It works just fine for me. What are the steps that you did?

I use nix so:

1. I installed OBS with all the plugins especially obs-vscapture
2. Run OBS and add Game Capture Source
3. Using lutris I enabled gamescope and changed the game resolution to something higher. I have a 1440p display so I tried 2560x1600 (16:10) to make sure I’m capturing it right.
4. Inside Lutris System Options game options tab I added OBS_VKCAPTURE and set it to 1.
5. When I run the game I can see gamescope is working as it’s projecting a 16:10 game onto my 16:9 screen.
6. Inside OBS I can see the game capture is working and when I select the Game Capture source it shows that it’s a bit too tall for my display.

Should be a similar thing for you, I would try something obvious like 16:10 first so you can get immediate visual feedback and then tweak your settings from there.

did you try this?

https://github.com/nowrep/obs-vkcapture

you need to capture the game not the gamescope instance.

You can use gamescope to set a higher or lower resolution and it’ll down or up scale it to your screen.

I presume you can use obs or something like that to record the raw footage before scaling.