It could be a stupid question, but what about compatibility mode? Does it still exist?
In W7 I always solved the problems I had using compatibility mode with old games...

Yep still exists and now has an option to run it in 16bit colour and not just 8bit colour like Windows 7 did. NT4 and 2000 are no longer compatibility options but I cannot really imagine those being used very much.

On the question of compatibility with gog games. The only one I had issues with was Journey Man Project 2 but I don't believe that is officially supported on windows 7 either. It is is still playable on 8 but nearer the end after half way you need to save a lot so as not to lose your progress. (Some people would of found this really frustrating but I guess I was able suffer through it without complaining too much)

Windows 8 is basically Windows 7 with the added "metro" part and some minor tweaks, the big majority of the thing that worked on Windows 7 should work on 8 including most drivers.

Anybody who is playing a game on Win 8 should put that info on the GOGWiki

http://www.gogwiki.com/index.php?title=Main_Page

I haven't played any GOGs yet, but so far I didn't have any compatibility issues with any kind of software/games.