With Steam, I haven't seen a game that doesn't work yet. I was just playing Clair Obscur yesterday, it worked great. I don't know about Gamescope, but I think you can run whatever Windows thing you want through Proton and it'll probably work well.
It’s been hit or miss for me. I mostly play strategy games. I often get weird game graphics, flickering and things like that that go away when I give up and reinstall windows.
