Gastrodon can counter some top tier pokemon like special dragonite. Gastrodon is usually seen on a rain team, but can function in the sun as well.Ok, yeah I would agree with that. But I would say that about any Water/Ground type, since that can sometimes screw with your team if you don't have a grass type. Quagsire is in the same boat, and he's in D tier. Like I said before, so many teams have Grass types that make Gastrodon irrelevant. Without a Storm Drain boost (and even with one), Gatrodon can't 2HKO Celebi (even offensive sets) or Venusaur, and can't OHKO Breloom. Then he just becomes setup fodder for them. Plus, most don't have Scald/Ice Beam/Earth Power/Recover because Toxic is usually in there somewhere which means that you're adding to the number of things that can beat Gastrodon. Obviously you won't know which moves your opponent is running, and playing carelessly could cost you, but I don't think its offensive (or even really defensive) presence is great enough to warrant B tier. Oh, and Psyshock from Life Orb or Specs Latios is a clean 2HKO, as is Draco Meteor from the Specs version, so it isn't really a good counter to him.
Grass types are the bane of water/ground types, but the only common ones can easily be handled by sun teams. Some water types have to run a grass type move just to get past gastrodon. Access to recover give gastrodon an edge over other walls like ferrothorn and forretress as he can stay in battle longer. Pairing him up with a wall like ferrothorn can give your team the ability to beat most special attackers. Gastrodon is a very useful pokemon if used correctly.