Windows 8 currently supports 90% of GOG's catalogue and that number will only rise as the code ninjas over at GOG continue to work into the night.
"[A]s of today we're adding official Windows 8 support for most of the games in GOG.com catalog. There are currently 431 titles fixed, tested, and reported to be working properly under Microsoft's new OS. Note that most of these have not had the master builds updated, so you shouldn’t need to redownload the installer or anything."
Making 90% of their catalogue compatible.
Finally some good news about the new OS, eh? What with Directx 11.1 being a platform exclusive, the kerfuffle surrounding 18-rated games, Croteam calling the OS a "Walled garden", Valve stating support for Linux, and Tim's less-than-fun experience with it.