Firstly, Gastrodon is Low C. Major difference. Secondly, what are the definitions for C and B again? Gastrodon is perfect for C rank. It has specific niches and viable defensive capability, but doesn't have the large offensive or defensive capability that B-rank requires. The things you stated, in reality, are just about the only things it counters, and even then it's still dependent on the set they run. Plus, you could put in Scarf Palkia to counter almost all of the things that you stated instead of Gastrodon, making Gastrodon sort of outclassed, making it even more fitting to have it in C rank. Just my 2 cents.