Starkrun: Ive been running window 8 since release candidate and every GOG Ive tried works.. even sanitarium..
Earlier someone here reported some issues with Gothic in Win8. Did you check whether it still has issues? I recall Diablo 2 was mentioned as well (but it is not a GOG).
When I recently tried the Win8 Preview, of the few (newer) games I tried, Arkham Asylum wouldn't start at all, and Crysis 2 had severe graphics problems making it unplayable. I don't know what the issue with AA was, but according to Google, Crysis 2 case was apparently some DirectX11 issue in Win8 preview, and some got it working by forcing it somehow to use DirectX9 instead, I think.
Crysis 1, Bioshock and Black Mesa ran great though. The performance seemed exactly the same as in Win7, also according to 3DMark11 (I've sometimes read some claiming Win8 would have better performance than Win7, I dunno what that means, at least I didn't see it). My tests ended when the Win8 partition corrupted my Win7 partition. :(
Since I am eligible to the 15€ Win8 Pro update, I think I will buy it, just in case. I will probably not install it, not until something comes that really requires or lures me to upgrade from Win7.