Have you tried it? I just installed gamescope and launched some games with it via steam launch options. So far I feel like the frame times are better. I use it on top of KDE and with an AMD card. Freesync in Gamescope works too. (Have it allways on in KDE if that matters).
No. Don’t really know what it’s supposed to do. I could use it to limit the framerate, but I already use mangohud for that and that works just fine for me. Maybe I’d use it to use FSR on a game that does not support it.