On the topic of Gastrodon, he's really not strong enough defensively to tank very many hits despite being slow while he also doesn't have a high offensive stat. His only real perk is Storm Drain, but even that is counteracted by the fact that many of the teams using water attacks for damage will also have Ludicolo or a grass type to be out on the field when they do that to help against bulky waters and tank Surf. Even in Rotom-W's case, if you're not running HP Grass on Gastrodon you don't really have a serious way of hurting him, so he isn't a very strong counter. Although, he is nice for what survivability he has and his straight up blocking of water abilities, I think his base stats just really aren't good enough for him to be one of the most effective 'mons in the game-especially in such a competitive tournament. But hey, I'm probably wrong. It's just I don't like it when my Gastrodon is 2hko'd by Metagross' Zen Headbutt because I built it to be able to tank a Latios Draco Meteor.