Wild Cards grew from a roleplaying game you ran. Did you and your fellow writer-participants stop gaming in the setting when you started turning it into a book?

Yes. It was too confusing to do both at once.

And speaking of gaming, I know you still take part in regular gaming sessions. Is there a main campaign/setting that's played primarily, or are you guys sticking to shorter campaigns these days?

Well, we played in Walter Jon Williams' Rome game (GURPS) for a decade or more, but Walter hasn't been running that lately. More recently we're playing a space game run by Ty Franck, which uses a D20 system. Ty's game is great fun, but frankly I don't like D20 nearly as much as GURPS. (Old GURPS, 3rd edition, at least)