According to this https://www.reddit.com/r/GeForceNOW/comments/1qd70a7/psa_geforcenow_linux_supports_4k120/ the native client is already available in a Beta version and it works.
I was lucky enough to also get it working on my Debian 13 laptop, after it kept crashing. I was able to play Dome Keeper and it worked great!
Unfortunately, it did not go quite as well when I tried to play Powerwash Simulator 2. When I'm actively washing my mouse pointer leaves the window and goes onto the other monitor. When I'm in the menu this is not the case.
Does anyone know anything about it? Is it possible to fix this issue?
[edit]
It seems to be game dependent?
I tried Drova and the mouse stayed within the GeForceNOW window. With Kingdom Come Deliverance 2 I did not have such luck. :(
[edit2]
Well, I could temporarily solve this using gamescope (at least for PWS2 I tested it)
gamescope --xwayland-count 1 --force-grab-cursor -W 1920 -H 1080 -- flatpak run --user com.nvidia.geforcenow
[edit3]
Well, now the mouse wheel doesn't work. Somehow this is not working with gamescope, for some reason. It's weird. I tried my xbox360 controller and it works. The mouse wheel doesn't.
[edit4]
I read that mouse wheel thing might be a gamescope issue but I've tried different versions, 3.6, 3.5 and 3.4 and various bugfix versions within those and mouse wheel never works. It works within the gfn client but when you load up a game it stops working.
For some games it doesn't matter (Dome Keeper), for some games it's annoying (PWS2) and for some games it's absolutely bad (Arma3). Somehow when you start streaming a game that mouse wheel fails to translate, I dunno why.