Ohoho, I'm sure you remember me!
- Your team, as you pointed out yourself, really is death fodder against Trick Room teams (hehehehe). To combat them, your best bet is a very defensive Pokemon; Gastrodon alone doesn't do the trick as well as some other Pokemon could, in my opinion. (I'm looking at you, Blissey.) You also have a crapload of offense, and you can afford not to constantly have your foot on the gas, so that's where I think Blissey could come in. It's pretty flexible.
- Should, however, you be adamant on keeping Gastrodon, my point about "crapload of offense" still stands, as with a boost from Storm Drain (which if you're smart, and I know you are) Gastrodon really only needs two of those moves to sweep. Thus Stockpile can make it a truly defensive Pokemon with a healthy dose of HP and access to Recover, and for that I recommend Stockpile. Thus I recommend replacing Scald, as Ice/Ground coverage is very favorable, and against rain teams, which Gastrodon is evidently included to counter, Scald will barely do anything that the other moves can not do.
I love this team. Although I've had my fair share of victories against it, it's still a royal pain in the ass in my books.
tl;dr:
You can afford to ramp up the defensive aspect a li'l bit, and with it you're golden. As if the team needed any further improvement.
9.5/10