Gastly is a common encounter in Sprout Tower at night. It can also be found in the Bell Tower and in the Safari Zone at nighttime. Haunter can be found on Route 8 and in the Safari Zone at night.
Gastly and co. are one of the staples of the franchise; they were the only Ghosts in RBY, and have been mainstays ever since. Unfortunately, for the first three generations, they were kind of terrible, since all Ghost and Poison moves were physical, and special attacking is their forte. However, Generation IV rolled around, and suddenly they became amazing: fast, hard hitters that decimated everything in their paths with a choice selection of moves. Of course, this is talking about competitively... in terms of Nuzlockes, the Gastly line is somewhat lackluster, mostly because of their horrid level-up movepool and defenses about equal to those of a wet napkin. However, raised correctly, these spooky specters can be veritable wrecking balls, able to smash everything that dare get in their way. Just keep them away from any Proton packs.
Gym #1 - Falkner (Violet City, Flying-type): Gastly is neither here nor there for this battle. Pidgey literally cannot deal damage to it, but Gastly can't damage Pidgey either without resorting to Curse. Pidgeotto, of course, is carrying Gust, which will quickly blow Gastly away, especially if you decide to halve your HP with Curse.
Proton (Slowpoke Well): Gastly's actually quite useful for this battle. Zubat only has Leech Life to attack with, which Gastly quad resists, although Supersonic can be irritating. Koffing, likewise, only has a piss-poor Smog to work with, but repeated SmokeScreens can screw up Gastly's accuracy and make the battle drag on for far longer than it should.
Gym #2 - Bugsy (Azalea Town, Bug-type): Gastly does extremely well in this battle, doubly resisting Scyther's U-turn while being immune to Quick Attack. You may find yourself in trouble if you let it get off some Leers and/or Focus Energy, but you should be able to hit it with Hypnosis with it by then and switch out to a teammate who can deal decent damage to it. Thankfully, a level 19 Gastly avoids the OHKO from a crit U-turn even if it's at -1 Defense; once it U-turns out, you can exploit the fact that Metapod and Kakuna are wholly pathetic and Gastly can kill them easily.
Rival (Azalea Town): Nope. Zubat has Bite to murder your face off, Gastly mirror-match is a bad idea since it also has super-effective Lick and can Curse-trap you, and the starters are packing real power and strong attacks, although Bayleef will probably lose if you have enough Potions and enough patience.
Gym #3 - Whitney (Goldenrod City, Normal-type): NOPE. Seems like Gastly would be useful here, since Normal gym and all, but Miltank has Scrappy to Stomp Gastly's ectoplasm into the ground. The best chance Gastly has to be effective here is hit the Clefairy with a Hypnosis, but just be wary of Hynosis' shaky accuracy and Clefairy can always Metronome into a move that deals heavy damage to your gaseous comrade.
Rival (Burned Tower): Same as last time, except now he also has a Magnemite, and you still shouldn't try to fight that with Gastly. Its ThunderShocks will be too powerful.
Gym #4 - Morty (Ecruteak City, Ghost-type): Gastly should be evolved by this point, at least into a Haunter, potentially a Gengar if you feel like trading directly after evolving. Still, this is a no with a capital NO. Your best Ghost move will be Shadow Punch; theirs will be Shadow Ball. Even the opposing Gastly will inflict huge damage with super-effective STAB Shadow Ball off of 100 base Special Attack. Seriously, just bring at least one of Raticate, Fearow, Persian, or Togetic with Extrasensory, and you win. No reason to risk Gengar's lack-of-neck.
Eusine (Cianwood City): Drowzee takes a massive crap on Haunter/Gengar unless you're packing Shadow Ball. Haunter can be easily dispatched since its strongest Ghost-type move is Lick. Electrode, however, should be avoided, as it's packing a strong STAB Thunder that it likes to spam.
Gym #5 - Chuck (Cianwood City, Fighting-type): Gengar can be decent here, IF (and only if) you manage to stall his Primeape and Poliwrath out of non-Fighting attacks (Rock Slide and Surf, respectively). Otherwise, they will leave huge dents in Gengar and probably 2HKO it.
Gym #6 - Jasmine (Olivine City, Steel-type): Once again, no. Thunderbolt from either Magnemite is going to hurt, and Iron Tail from Steelix is a two-shot if you're lucky (a OHKO if you're not). Your best attack against them would be Focus Blast from the Goldenrod Department Store, and the accuracy on that is shaky at best, even with the Wide Lens equipped. Keep away.
Petrel (Team Rocket HQ): Beware of Raticate's Crunch. While Thunderbolt, if you already have it, can make short work of it depending on your level, Haunter should not attempt this. On the plus side, Koffing almost can't touch Gengar at all, and Haunter has a good matchup against it as well. Zubat is plain laughable.
Ariana (Team Rocket HQ, tag battle with Lance): Not wise to lead with Haunter or Gengar here. Arbok has Crunch, and Drowzee, while being very low levelled, has Confusion. Gengar can take out one of them, but not both, and Dragonite will likely use Fly. Another problem later down the road is Murkrow's Pursuit. Assuming Gengar would fight something other than Arbok and Drowzee, it would have to bail before Gloom is taken out, as that's when Murkrow will enter the field. Overall, not a good idea.
Gym #7 - Pryce (Mahogany Town, Ice-type): Assuming you picked up a Thunderbolt TM from the Game Corner, Seel and Dewgong will go down easily. However, Piloswine has a ton of bulk and STAB Ice Fang with which to kill your ghastly friend. You should have Shadow Ball from level-up at this point, and you can get a Sludge Bomb TM in the route before the Lake of Rage to teach to Gengar as well. If you're feeling lucky during this battle, keep Gengar in against Piloswine and try to hit a Focus Blast. If you land one, great; it'll probably be a OHKO. If it misses, then Ice Fang from Piloswine will probably score a 2HKO or 3HKO, depending on how bulky and how high of a level your Gengar is at the time.
Petrel (Goldenrod Radio Tower): Gengar is immune to their bomb moves and doubly resists Poison. Go crazy.
Rival (Goldenrod Underground): Gengar's actually useful for this battle, since it's fast enough and has the moves to kill Golbat, Haunter, and if you didn't choose Cyndaquil, Meganium and Quilava. Don't even try Magnemite (if you're lacking Focus Blast), Sneasel, or Feraligatr (has the ability to survive a Thunderbolt), since they will all murder Gengar in short order.
Proton (Goldenrod Radio Tower): Again, Poison-types. Gengar is good at this. Fry Golbat with a Thunderbolt and use Shadow Ball to defeat Weezing, simple as that.
Ariana (Goldenrod Radio Tower): Arbok once again has Crunch. Unless you're way overlevelled or have the Choice Specs handy, don't try fighting it; it has enough special bulk to take a Shadow Ball. Granted, it shouldn't kill Gengar in return, but it's not worth risking it. Vileplume is easy enough, but Gengar will likely have to withdraw before it's taken out, unless you're playing on Shift mode, as Murkrow still packs the pesky Pursuit.
Archer (Goldenrod Radio Tower): You can kill Koffing easily, but Houndour and Houndoom are far too risky; Faint Attack and Bite will deal horrific amounts of damage to the poor little phantom. Don't bother.
Gym #8 - Clair (Blackthorn City, Dragon-type): Basically, you can kill Gyarados with Thunderbolt, but NOTHING ELSE. Kingdra's Hydro Pump and the Dragonairs' Fire Blasts and Aqua Tails will kill Gengar easily. Stay away from this battle if at all possible.
Kimono Girls (Ecruteak Dance Theater): Gengar has a bad matchup against Umbreon, and a very dicey one against Espeon, which is a speed tie and not a guaranteed OHKO. Choice Specs Shadow Ball will do the trick for sure, though, if you know you can outspeed. Flareon, Jolteon and Vaporeon are much less problematic, but will likely all need healing support, save for Flareon.
Ho-Oh (Bell Tower, HeartGold only): Sacred Fire from Ho-Oh is quite strong, and nothing Gengar wants to mess with. However, it's also not easy finding something that can take a Sacred Fire to the face without faltering. In a pinch, Gengar can about 3HKO Ho-Oh with Thunderbolt, or 2HKO with Choice Specs, if you manage to keep it alive throughout.
Lugia (Whirl Islands, SoulSilver only): Not worth it. Lugia's Special Defense will make it survive a Shadow Ball and while its Extrasensory is not as bad as it sounds, it undoubtedly has an advantage over Gengar's much poorer defensive power.
Rival (Victory Road): Kadabra can be handled, Golbat and Haunter are also easily defeated with the right moves, and you resist all one of Meganium's attacking moves, but everything else will prey on Gengar's low defenses. It is recommended to keep Gengar away from this fight unless you've got a very large level advantage.
Elite Four Will (Indigo Plateau, Psychic-type): Will's a bit of a risk. Gengar's weak to all the Psychic attacks he'll inevitably sling around, but at the same time Shadow Ball is going to leave very large, very noticeable holes in everything as well. It really depends on how overlevelled you are for this fight; make an executive decision. Unless you have a 0 Sp. Attack IV, 0 Sp. Attack EV, Adamant Gengar, though, you probably won't have any trouble mowing through Will's team with STAB super-effective Shadow Ball off of Gengar's insane base 130 Special Attack. You should definitely equip the Choice Specs, however, so that you ensure that Gengar KOs what it needs to KO.
Elite Four Koga (Indigo Plateau, Poison-type): Gengar resists all of the Poison attacks flying around, but can't really do much in exchange unless you choose to run Hypnosis/Dream Eater. Basically, hammer away at everything with any super-effective moves you have (Shadow Ball otherwise), but don't risk Venomoth smacking you around with Psychic. Crobat's kind of a gray area, as it can be OHKOed or 2HKOed by Thunderbolt but also has a powerful STAB Wing Attack that might 2HKO or 3HKO you in return.
Elite Four Bruno (Indigo Plateau, Fighting-type): Gengar does amazingly against his Hitmontop, because it can't damage him. Everything else, not so much. Onix has shit for Sp. Defense and a Shadow Ball will probably kill it, but Blaze Kick from Hitmonlee, the punches from Hitmonchan, and Rock Slide from Machamp will be pretty annoying. Fortunately, Shadow Ball should be able to score 2HKOs on all three of those things, so if you're at full health, you might want to whittle away at those pesky Fighting-types, since none of them have any extremely powerful moves that they can hit you with.
Elite Four Karen (Indigo Plateau, Dark-type): Umbreon and Houndoom are extremely tanky and powerful, respectively, and should be definitely be avoid. If you have NOTHING else that can handle Houndoom, you can attempt a Choice Specs-boosted Focus Blast, but really this strat is highly risky. Murkrow, despite having relatively low Attack, can smack you around with a STAB supereffective Sucker Punch. Thankfully her other two mons aren't too bad to handle: spam Specs Shadow Ball against Vileplume (although be cautious of its Petal Dance) and the Gengar (which thankfully does not have Shadow Ball in return, but do watch out for Destiny Bond).
Champion Lance (Indigo Plateau, Flying-type): Gyarados gets rekt by Thunderbolt, but the Dragonites are too dangerous for Gengar to handle. Aerodactyl and Charizard are both weak to Thunderbolt, but they're also both relatively bulky and may not go down to a single attack, leaving them able to spam powerful moves that could very well OHKO Gengar. In Aerodactyl's case in particular, Speed is also a problem. You may want to equip the Choice Specs and defeat Gyarados and Charizard, though giving up on Aerodactyl in the process, unless you have a nature that is beneficial to Speed and/or are at a substantial level advantage. Try using a Golem or something against those things.
From this point onwards, you can fight the gyms in any order, though you will need to retrieve the Machine Parts from the Cerulean City gym before you have access to the earlier portion of Kanto. Feel free to anticipate or postpone any battles as needed.
Gym #9 - Brock (Pewter City, Rock-type): If you taught Gengar Giga Drain, this gym is a cakewalk. If not, the two fossils will go down to Thunderbolt and the rest should be dealt with thanks to Shadow Ball. Levitate helps greatly here.
Rival (Mt. Moon, optional): Alakazam is more problematic now that it has evolved. It might survive a Shadow Ball and/or outspeed, depending on your stats. If you're certain to outspeed and/or OHKO, use the appropriate Choice item to get the rest done. Golbat and Meganium will go down about as easily. Gengar is a mirror match; you should be at a higher level, so go ahead. Avoid the rest.
Gym #10 - Misty (Cerulean City, Water-type): Golduck has Psychic, though if Gengar's Thunderbolt doesn't kill, Golduck shouldn't kill either. Thunderbolt also shortcircuits Lapras and Starmie for good (note that Starmie has no Psychic STAB). Quagsire isn't threatening at all, be patient and wear it down with Shadow Ball, or go with Giga Drain.
Gym #11 - Lt. Surge (Vermilion City, Electric-type): Not much of anything here can hurt Gengar. Spam Shadow Ball until everything dies. Choice Specs recommended.
Gym #12 - Erika (Celadon City, Grass-type): Either use Sludge Bomb or, if for some reason you don't have it, Shadow Ball. Then watch everything fall to pieces, including Victreebel, despite the lack of weakness.
Gym #13 - Janine (Fuchsia City, Poison-type): One of the Ariados and Venomoth have Psychic. Ariados has crap Special Attack, so it won't be a problem, whilst Venomoth packs quite a punch. If possible, leave it to something else. Crobat goes down to Thunderbolt, and Gengar laughs at Weezing's Explosion.
Gym #14 - Sabrina (Saffron City, Psychic-type): While Shadow Ball is a major winner here, Sabrina's Pokémon can also hit for super effective damage. Equip a Choice Specs and make sure you can outspeed both Espeon and Alakazam. Espeon has the same base Speed as Gengar, so it shouldn't be an issue bar Speed-lowering natures, but the boost - or a higher level - is needed for Alakazam. The Choice Scarf is an option, but do your calculations first, and avoid fighting anything you can't OHKO; they might just score a critical hit and OHKO you back.
Gym #15 - Blaine (Seafoam Islands, Fire-type): Overheat is nothing to sneeze at, and all of these Pokémon have it. Gengar should be able to dispatch Magcargo before it does anything, but Magmar will probably survive a hit and set up Sunny Day. Don't fight Rapidash, its physical offensive is really strong.
Gym #16 - Blue (Viridian City): Exeggutor goes down to Shadow Ball, and both Gyarados and Pidgeot are easily electrocuted by Thunderbolt. Rhydon and Arcanine need better counters, as their physical moves can greatly hurt Gengar. If you have Psychic, go ahead and fight Machamp; if not, avoid it.
Rival (Indigo Plateau, optional): Crobat is evolved but still falls to a couple Thunderbolts. Gengar can, as per usual, make short work of the rival's own Gengar and possibly his Alakazam, depending on your Gengar's level and item choice. Meganium is easy too. Once more: don't fight anything else.
Red (Mt. Silver): Gengar can outspeed and OHKO Pikachu easily. Venusaur also doesn't have much to damage it with, so Shadow Ball should make short work of it. Charizard and Blastoise are tougher, especially after setting up their respective weather moves; try to defeat them as soon as possible with Thunderbolt. Lapras suffers Thunderbolt, but can also hit back with Psychic; tread carefully, if you must. Lastly, as much as walling Snorlax seems like a good idea, it's actually really bad, because it has Crunch. Never come close to Snorlax.
Gengar's level-up moves are as bad as the grinding in Johto is. Hypnosis is decent, even with terribad accuracy, but Lick, Spite, Mean Look, Shadow Punch, Payback, and Sucker Punch are all horrid and not worth keeping for too long. Confuse Ray is also not a great idea due to its unreliability; one wrong move and Gengar's got a face full of super-effective move. Curse can potentially be useful early-game, but only if 1) you're certain the opponent cannot 2HKO you and 2) you have plenty of healing items. If you're looking for a less risky early-game option, then Night Shade is the way to go. Shadow Ball comes at level 33, and it is the BEST move Gengar learns by level up; if you're impatient, you can also use your TM from Ecruteak on it. Dark Pulse is redundant and lacks STAB, Destiny Bond is useful when you get killed (read: what you're trying not to do). Nightmare might sound decent in theory, but in reality you have to keep using Nightmare during each individual sleep cycle that the opponent goes through. You might as well use attacking moves instead.
The TM moves are another story altogether. Thunderbolt is a must on Gengar; get a TM from the Game Corner, you won't regret it. Sludge Bomb, which you get on Route 43, is the only Poison move Gengar will learn; it's worth it. If you get lucky enough to snatch a favorable Hidden Power (Fighting, Ice, or Fire) then that's always an option. Focus Blast is a great option and technically provides perfect coverage along with Shadow Ball; however, just know that if you miss Focus Blast at the wrong time (i.e. against move Steel or Dark-types) then you could very well wind up with an (un)dead Gengar. If you plan on doing the postgame, then Psychic, Energy Ball, and Giga Drain can all argue for moveslots.
Minimum stats are calculated with 0 EVs, IVs of 0, and a hindering nature, if applicable.
Maximum stats are calculated with 252 EVs, IVs of 31, and a helpful nature, if applicable.
Gastly Line Ratings
What Nature do I want? Anything that's not -Special Attack is decent. Modest or Timid are wonderful, but Gengar can work with most Natures.
At what point in the game should I be evolved? Before Morty.
How good is the Gastly line in a Nuzlocke? Gengar kinda gets a raw deal in Johto. Most gyms have some way of murdering it, there's Dark coverage everywhere, and low defenses don't help its case. However, it is certainly worth the time and effort you have to invest in it. Ghost-types are a rarity in Johto (just them and Misdreavus, who, uh... yeah, no) and Gengar is a great one, even if they often end up dead due to their being made of paper. Still, high Speed, amazing power, and great coverage; overall, a solid, yet gaseous, Pokémon.
Weaknesses: Psychic, Ghost, Dark
Resistances: Grass, Poison (x0.25), Bug (x0.25)
Immunities: Fighting, Normal, Ground (Levitate)
Neutralities: Rock, Steel, Dragon, Fire, Flying, Ice, Water, Electric