Hi there, just noticed this announcement and although it is a year later, I can't restrain from advertisement although it is completely unrelated. I am working on Websockets + EventMachine + CoffeeScript engine that can power any MTG like game (so far only MTG). If you want to take a look, see here:
https://github.com/HakubJozak/manaKaerdon, if you want to develop a card game (and provided you still work on that) but don't want to hassle with the programming, you can fork the repo or help to generalize it so that it covers your game too and you can concentrate on what's important (the GAME). I guess the benefit for card game running in the browser are huge and does not really make sense to do it in GOSU. But if you just like GOSU and have fun working with, go for it!
(btw don't take it the wrong way - Gosu and Chingu are cool: big fan ;] - just don't think it is the right tool for the job)