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).
I use it on Valheim to properly capture the cursor, as it will otherwise keep escaping to my other monitor during build mode or similar situations.
I’ve actually invoked it to watch older videos, enabling FSR to add a bit of extra sharpness.