I have killed many Gastrodon in my laddering career. If that's the best you can do for Thundurus, then you're probably going to get swept, since Life Orb variants can +2 Focus Blast into HP Ice for an easy 2HKO. Lum variants have to get two Focus Blasts in, which is easier said than done, but it doesn't make Gastrodon any better a counter.
And Quagsire is still not being used. Fancy that.
Judging from some of my experiences against Thundurus users, I can earnestly see why some people do not find it a big deal. People seem to really enjoy leaving it in on my naturally slower and obviously scarfed Hydregion/Politoed/whatever. You can switch out of that, you know. Once he's back in, guess what's not coming in safely. Yes, that Rotom-W that gets OHKOed by Thunder after rocks. I've used Mamoswine and Scarf Landorus to switch in on Thunder, Starmie to switch in on Focus Blast, etc. These are not reliable techniques to beat Thundurus. In fact, most of my "sweeping" from Thundurus gets done just throwing around attacks. Like that clever Excadrill switch-in players like Oz don't think I see coming. Suboptimal strategy shouldn't give overpowered Pokemon a free ticket into OU.