To play Proton games you only need the latest gamescope from git, the HDR layer and environment variables are no longer needed for gamescope
Just set the launch arguments on Steam for any game to: gamescope --hdr-enabled --nested-refresh 165 --fullscreen --steam -w 3440 -W 3440 -h 1440 -H 1440 – %command%.
Don’t forget to set your refresh rate and resolution.
If you want to play videos files in HDR (YouTube also works) you need to use mpv together with the HDR layer.
After installing the layer you can run mpv like this: ENABLE_HDR_WSI=1 mpv --vo=gpu-next --target-colorspace-hint --gpu-api=vulkan --gpu-context=waylandvk “https://www.youtube.com/watch?v=dQw4w9WgXcQ”.