Gastrodon

[Glitchwood High]

Base Stats: 111 HP / 83 Atk / 68 Def / 92 SpA / 82 SpD / 39 Spe
Abilities: Sticky Hold | Storm Drain | Sand Force (Hidden Ability)
Notable Moves:

• Earthquake​
• Ice Beam​
• Chilling Water​
• Surf​
• Recover​
• Protect​
• Stealth Rock​
• Spikes​
• Yawn​

Pros:

• High special defenses in tandem with it's reliable recovery and lack of weaknesses make it a sturdy wall versus the likes hard to check Pokemon like Hydreigon, Kilowattrel and some Gengar variants.
• Water immunity is especially helpful in rain matchups
• Can set hazards reliably.
• Chilling Water and Yawn can make life difficult for switch-ins.

Cons:

• Gastrodon tends to be very passive and gets rid of a lot of momentum upon being switched in.
• It is very susceptible to being abused by setup sweepers.

Terastalizing Potential: Gastrodon is one of the least terastalization-reliant Pokemon in the tier, but when needed there are a handful of types it particularly benefits from.

• Tera Flying is by far the preferred choice, as it makes Gastrodon easily counter Barraskewda and check Slither Wing, two Pokemon that could otherwise threaten or outright force out Gastrodon. It also allows Gastrodon to resist normally quadruple-effective grass type moves, and the ground immunity is well-appreciated.
• Tera Poison has the same resistances to Fighting and Grass but the electric neutrality can be helpful in certain matchups.

Potential Sets:
Standard SpDef Wall
Gastrodon @ Leftovers
Ability: Storm Drain
EVs: 252 HP / 4 SpA / 252 SpD
Calm Nature
IVs: 0 Atk
- Recover
- Earthquake
- Chilling Water / Surf / Ice Beam
- Spikes / Stealth Rock / Yawn / Ice Beam

This set is straightforward yet has a lot of flexibility in the last two moveslots. It warrants it's large special defense to switch into a huge amount of special attackers and starts causing earthquakes. Gastrodon can setup either Spikes or Stealth Rocks to utilize free turns, but Yawn can really effective for scaring away physical attackers and potentially racking up hazard damage through repeated switch-ins. Ice-coverage can severely threaten several different flying-and-dragon types, making it less passive through increased damage output. Finally, chilling water is significantly weaker than surf but has far more utility. Gastrodon can work with either a three-attacking set if it's team already has great hazard-stacking, or two attacks for defensive utility.

Other Options:

• Protect makes Gastrodon horrifically passive but can be incredibly annoying versus choice-locked attackers, especially Slither Wing.
• Clear Smog is a niche option but it stops Espathra dead in its' tracks and is fantastic versus hyper-offense.
• Sludge Bomb can provide invaluable utility by poisoning Pokemon like Talonflame, though it lacks consistency in comparison to ice beam.

 

[Celebiii]

I'd like to suggest a fully physically defensive spread as an option. I also think you can drop STABs on Gastrodon entirely and run Sludge Bomb + Ice Beam on certain sets. If running only special moves, Gastrodon can also use Earth Power instead of Earthquake if needed. Finally, Sticky Hold can also be run on Gastrodon to take advantage of the fact that Storm Drain is so expected, although once the lack of a Water immunity is revealed the Gastrodon user needs to be careful with it.

 

[STEELDRAGON]

I have found Gastrodon to a valuable special wall and one of the best answers to rain teams. What's nice about is it is it isn't super passive either and can hit kinda of hard at +1. I generally run all damaging moves as special moves due to them hitting almost everything harder except maybe TInkaton, and the prevalence of Intimidate users. Losing Toxic is a bit of a hit but still performs pretty well.