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 straight up consider Gamescope Linux’s killer app and almost the primary reason I prefer to game on linux now as someone who plays older games with resolution and alt+tab issues and also has a steam deck where wrangling resolutions in a certain oddball way matters for performance reasons.
I just wish it worked on Nvidia better. It used to and I loved it, but I’m having the same issue as hopeisforpresidents where it just gives me a black screen when I try to use it on desktop now.
But yeah when it works, it’s very nice, especially for old 640x480 point and click games that have pre-rendered assets that scale nicely with FSR 1.0.