In my experience, there's very few games that can't be tweaked to work with Win 10 on here. It's more compatible with some older games than Win 7, it's just that you have to manually tweak the games, and most people can't be trusted to do that ;)

I remember that happening in the past, like for FlatOut (once they properly got rid of the copy protection completely, which was causing problems especially in Windows 10), and e.g. Haegemonia Gold Edition, which earlier wouldn't work on anything beyond Windows 7 (I know because back then I reported having problems running it in Windows 8,1, and they replied sorry it isn't supported on anything newer than Windows 7; now it is, including Windows 10, so they somehow figured it out how to make it work).

I'm sure there are lots of other games too which got the Windows 10 support later. As said, for many of them the case probably just was that GOG hadn't yet just tested it completely on Windows 10, but in some cases they have added extra fixes too to make it work there (like those aforementioned games).

The best place to check for Windows 10 compatibility for the unsupported games, is to visit said games' subforums here on GOG. Usually they are full of threads about issues and possible fixes for Windows 10 compatibility. Which reminds me, does Alone in the Dark The New Nightmare already work in Windows 10, maybe with dgVoodoo?