Actually, Gastrodon can be a very good Pokemon in the right hands. It's ability Storm Drain makes it a good Fire or Ground-type partner in doubles and even in singles, it's brilliant bulk lets it act as a special wall and a staller Pokemon. It also has access to Acid Armour and Amnesia and Recover which make setting up a breeze. Combine with Clear Smog and you have an amazing tank that can set up on most opponents and stop them from setting up on you. Give it a Lum Berry to help stop someone from using Toxic on it or have another Pokemon use Safeguard or Aromatherapy/Heal Bell to cure it. Only way to take it down would be to either Critical hit it with a Grass type move, or hit it with one before it can set up, but avoiding that would be up to the skill of the player, and Clear Smog is Poison type so would destroy any Grass-types that tried to hit it after it had set up.