Gastrodon shines with Rotom-W around, being a good switch-in against the washing machine most of the time. He isn't outshone by RW either, Gastrodon has a role distinct from tanky RW. RW avoids being OHKOed by weak grass moves and has volt switch, but lacks reliable recovery with leftovers like Gastrodon's. Due to this, Gastrodon can stall and check better than RW, but has more likely counters due to energy ball/giga drain distribution and HP grass. Rotom is usually used to burn with a 100% success rate, while Gastrodon usually uses Toxic to stall and Scald to threaten physical attackers. Finally, Gastrodon likes sand far more than Rotom-W does.
According to the current tier list, Gastrodon is mostly useless against ~23/64 of current OU threats with right set, and can stand up to the most common sets of ~30/64, which I would call a significant portion of the metagame (the list is far from complete, of course) Just within S and A ranks, he is unable to reliably impact only ~16/40 threats, leaving him useful against 60% of the pokemon that should be above/near him on the rankings.
Gastrodon is A-, because he needs protection from grass moves, powerful normal/fighting/fairy/dragon moves, Toxic, Taunt, and sweepers who find him to be setup bait. However, he can go up against a decent portion of the metagame, especially a few very common pokemon.
Assuming this set:
Gastrodon @ Leftovers
Ability: Storm Drain
EVs: 216 HP / 252 Def / 40 SAtk
Bold Nature
IVs: 0 Atk
- Recover
- Scald
- Toxic
- Earth Power
When it comes to not-so-obvious OHKOS, Gastrodon can OHKO mega-lucario and terrakion after close combat, can OHKO landorus-I and Mega-Mawile after a storm drain boost, OHKOs offensive Aegislash after an attack because it is faster than Gastrodon, and OHKOs Talonflame after Brave Bird recoil.
Checks/counters the most powerful common sets from Rotom, Aegislash, M-Charizard-X, +1Atk Genesect, Terrakion, Heatran, Talonflame, Latias, Dragonite, Greninja, Excadrill, Keldeo, (M)Tyranitar, (M)Scizor, Landorus-T, Gengar, Gliscor, Klefki, Conkeldurr/Bisharp/Weavile if you can lose leftovers, and most defensive pokemon w/o grass moves. I ran a bunch of calculators to figure these out, but firefox crashed and I lost all the data. The things that really stood out were an ability to check +1 SpA Scarf or Lum Genesect sets without Bug Buzz and Aegislash being unable to OHKO Gastrodon at +2 Atk with life orb.
Pokemon without grass moves Gastrodon is scared of include Mega-Ampharos, Latios, Exploud, Azumarill, Mega-Mawile, Mega-Pinsir, Mega-Gardevoir, Mega-Medicham, Mega-Heracross, Diggersby, Honchkrow, Porygon-Z, Landorus-I, Kyurem-B, Mega-Garchomp, Crawdaunt, and Garchomp, usually only if they are wearing orb/specs/band for the non-megas. Of course, it's mostly useless against setup, taunt, and special walls that can avoid toxic (Deoxys, Thundurus-I, Blissey, Clefable, Scolipede, Nasty Plot/Calm Minders it can't 2HKO)