Gastrodon can be found on Exeggutor Island, in the Poni Wilds and Ancient Poni Path with a 10% encounter and 20% if you are surfing around Poni Wilds.
Gastrodon was known for being one of the most common Water Pokémon you could encounter in Sinnoh from the start of the game until the end. But this time, Gastrodon is only available in three areas from the fourth and last island, as well as having one of the lowest encounters rate from them. Being such a lucky encounter that comes at late game, you expect expect a world's destroyer, something that can really be at the same level as the other great Water-types that live in Alola, a Pokémon that is over monsters like Primarina, Drizzle Pelipper or Wishiwashi.
And while the answer to that questions is probably a no, Gastrodon has its own weapons, one of them being its buffed ability Storm Drain, one that in the past was even more pointless than Sticky Hold, is now the slug's main reason to be in your team.
Oh, and I hope that you love the blue color, because that's the only one you will get.
Hapu (Vast Poni Canyon, Ground-type): This is a really good battle to try your newly caught slug. Dugtrio hits hard, but not enought hard to be a problem and would go down to a single Scald. You probably want to switch to something that can take down Flygon in a hit as your best Ice-type move will be Blizzard and then continue with Scald, but in general, Gastrodon can deal quite well with Flygon. Mudsdale is a monster and that Z-move can OHKO the tankiest of the Pokémon, I suggest keeping Gastrodon and in the first turn switch to a Pokémon that you know that it can't be killed by the horse's Z-move before returning to Gastrodon after it was used. Scald is the word here, if you get a burn on Mudsdale, the rest of the fight is done. Hapu's Gastrodon doesn't have Storm Drain, so if yours owns that ability, then this battle is done as the AI can use a Water-type move at least once buffing your special attack.
Totem Kommo-o (Vast Poni Canyon): Clanging Scales is a really powerful move and as Gastrodon mostly hits with its special attack, you probably will end fighting both the Totem and the helping Pokémon. Better use someone that can OHKO the Kommo-o.
Lusamine (Ultra Space): Stay freaking away from Lusamine's Lilligant, I don't have to tell you the reason. For the others, Clefable has that nasty Metronome that can give you a lucky Water-type move to buff a Storm Drain Gastrodon or an unlucky Grass-type one that will kill it, the other reason is that the combo of Cosmic Power and Moonlight drags Gastrodon to a stall battle that will probably end losing. And talking about stall, let's meet Milotic, if you have Storm Drain, prepare for the longest battle in your life, Milotic can't hit you (Flail does nothing) and you can't hit Milotic hard enough to cover all the health Recover gives back to Milotic, so in the end, you will discover why stall is the most hated playstyle... or you will end loving it and start training a Celesteela for online battles, gladly, is impossible for Storm Drain Gastrodon to lose this match. Mismagius is a normal battle, easier if you come from Milotic after a Storm Drain buff and for Bewear, you only need to get a burn from Scald and call it a day.
Solgaleo (Altar of the Sunne, Sun only): Burn it. Burn this guy before it can't Crunch your slug to oblivion, the other choice is to use Mud Bomb to lower Solgaleo's accuracy while dealing Super effective damage. But really... BURN THIS THING... or just throw the Master Ball.
Lunala (Altar of the Moone, Moon only): Throw the Master Ball or lead with another Pokémon, you don't want to face it with Gastrodon.
Gladion (Mount Lanakila): All of his Pokémon are really weak to Scald's burn, with the exception of Fire Silvally, but is still weak to Scald. You can hit Crobat and Weavile with Rock Slide and Blizzard the first one and Hidden Power Fighting the second one, they will go down in few hits and you can always go with Scald for some stalling. Lucario is the easiest, you resist its Z-move and can't die to Aura Sphere, so use Scald or Mud Bomb to take it down. For Silvally, both Water and Fire can be fought by Gastrodon, while you must be away from Grass.
Hau (Mount Lanakila): Stay away from all the Grass-types in his team and the Komala as it knows Wood Hammer. Raichu hits hard with Psychic, but you can do the same with Mud Bomb, lowering its accuracy. With Storm Drain, both Vaporeon and Primarina become less problematic as the game can use at least a water move ones, Mud Bomb Vaporeon and take down Primarina with Toxic-Recover or Sludge Wave/Bomb. For Flareon and Incineroar, use the STAB Super Effective move of your like.
Elite Four Hala (Pokémon League, Fighting-type): All of Hala's Pokémon hits hard, like really really hard. You can use Hariyama's first turn to buff your Defense with a X-Defense and then continue with Scald, burns will be appreciated in this fight. Do the same with Primeape and Bewear, but not the Poliwrath as it has Water Absorb, for this one you will have to use Mud Bomb. In the case of the Crabominable, you should switch to something that can OHKO-it as its Z-move hits like a freaking truck. If you managed to stall one of Hala's Pokémon and use more than one X-Defense, maybe you can survive its attack at full HP.
Elite Four Olivia (Pokémon League, Rock-type): Storm Drain kills her. Her lead is Relicanth and it will use Hydro Pump whatever your ability is, giving your slug a free Special Attack buff to 2HKO and OHKO everything in her team with Scald or Mud Bomb. Your worst problem will be Lycanroc's Rock Climb's confusion or any move that lowers your Defense or Special Defense, but in general, this battle is too easy for Gastrodon, even the Sticky Hold one will end coming out victorious with little effort.
Elite Four Acerola (Pokémon League, Ghost-type): Both Palossand and Dhelmise knows Grass-type moves, so stay away from them. Sableye can be quite annoying with Confuse Ray and Zen Headbutt, but in general it's not dangerous enough to be a problem for Gastrodon, Scald and good bye. Drifblim on its own isn't a problem, but if it gets enough buffs with Ominous Wind and pass it to another member with Baton Pass, them... that's a great problem, in case of passing this buffs to Palossand, I hope that you have a Toucannon, because that thing coverage hits most of the Pokémon in the game. So yes, use someone that can take down Drifblim in less than two turns, can use Taunt or remove buffs. Froslass can use Confusion Ray, Shadow Ball can lower your Special Defense and Blizzard hits hard, but aside of that, the Ghost's average stat and Gastrodon's nice Special bulk, makes this battle not really problematic.
Elite Four Kahili (Pokémon League, Flying-type): Toucannon has Skill Link Bullet Seed, so keep Gastrodon back for that match. The rest of the battle can be described as annoying, none of the Pokémon have dangerous sets, but most of them pack confusion and the leading Skarmory likes to use Spikes. Ice Beam and Scald deal with all of them, but due to this annoying sets, I prefer to let you decide if you want to use the slug or go for a Pokémon you know that can OHKO them quickly.
Professor Kukui (Pokémon League): Decidueye is a Grass-type, you already know what to not do. Focusing on the rest of this great ches... I mean, great professor's Pokémon, you can OHKO Lycanroc with Scald and Magnezone with Mud Bomb. You can try to burn both Snorlax and Braviary, but you should leave another Pokémon deal with the first one and try to Ice Beam the second one. Ninetales hits hard, like really hard, so you probably want to have a Steel type dealing with this. You can bait Primarina to use its water Z-move and get a buff thanks to Storm Drain and hit it back with Sludge Wave/Bomb, for Incineroar use Mud Bomb to lower its Accuracy or Scald to get a 2HKO.
Tapu Koko (Ruins of Conflict): Spam Mud Bomb and done, you can catch it later when Nuzlocke rules aren't in play.
Post-Game: And if you are still Nuzlocking the game, please, think about it because the bosses here are freaking dangerous for any Pokémon you can have.
Gastrodon can come with the following moves Mud Bomb, Hidden Power, Rain Dance, Body Slam or Muddy Water if it has level 41 to 43. Mud Bomb is going to be your only Ground-type STAB in all the game due to Earthquake being postgame and Earth Power a Tutor Move. Hidden Power can be a good coverage move to cover holes, Fighting or Fire will be some good options. Rain Dance is not really worthy, Drizzle Pelipper is a thing in this region and it only works if you have a Monotype Water team. Body Slam has its use for Paralyzing your opponents, but in general, you may probably change it for more coverage. Muddy Water would be a great coverage option, but for the time you get Gastrodon, you already have Scald that is better. The only move Gastrodon will learn for the rest of the game is Recover at level 54, great for stalling with burn or poison and to save healing items like Max Potions.
None of the relearner moves are really amazing, so you aren't going to make use of it with Gastrodon. Now, for TMs, your first option should be Scald, Gastrodon has great bulk and being able to burn your opponents while dealing nice damage, is one of the best things you could ask for the slug. For coverage you have Sludge Bomb and Sludge Wave, both are similar, although Sludge Wave ends being a little bit more usable due to better power and area damage. Ice Beam is your generic ice coverage and while you don't have it, Blizzard can be usable too. Rock Slide may not be the best coverage move due to Gastrodon's slower attack, but still can help with Ice and Flying types. For funs of stall, Infestation is another TM option for Gastrodon, that paired with burn or poison from Toxic and the regeneration from Recover, can make the slug the child of the devil. Hidden Power can be recovered with TM saving Heart Scales for other members. Protect is a gimmick option if you really need someone that tanks Z-move as while you can't stop them with Protect, you can lower the damage heavily.
Pokémon that are good against Grass-types: Gastrodon has only one weakness, which is very easily covered for, since Grass-types have many weaknesses and can be hit for super effective damage by countless other types. This counters aren't rare, between snowy areas, caves and volcanos, you can find a lot of Pokémon that can deal with Grass.
Fast hitters: Gastrodon is, as you expect from the Slug Pokémon, slower than a Squirtle. For that, is nearly impossible for this line to hit the enemy without suffering any damage first, that way, any Pokémon that isn't OHKO-ed and can put Gastrodon near the 2HKO range is a problem. Fast Pokémon help with this, with high offensive and being able to outspeed most of their enemies, they can work along with Gastrodon and save you from some bad time.
A few examples of this include Crobat, Mimikyu, Ribombee
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.
Shellos Line Ratings
What Nature do I want? Anything that lowers Gastrodon’s speed. The slug has a really low speed, even for Alola, and most of the Pokémon in the game are going to outspeed it, so sacrifice the Speed to get either Defense (Relaxed) or Special Attack (Quiet); Attack (Brave) and Sp.Def (Sassy) are fine too, but the other two are better options.
Which Ability do I want? Storm Drain. Sticky Hold is pointless and you aren't going to get use to it probably in the whole game, but Storm Drain? Is not only that you get a second immunity that the game can't take into account, but the Special Attack buff you get from it is really helpful for Gastrodon's nice Special Attack.
How good is Gastrodon in a Nuzlocke? Gastrodon's main problem in this game is that it comes too late and only the ones that have lost a team member around Poni Island will think about adding Gastrodon to their team. But if you do? Enjoy one of the best Water-types in the game, with great bulk, an amazing ability in Storm Drain, recovery move, stall options, status and in general, really good matchups, make Gastrodon a nice addition to any team that needs a late game replacement.