Proper HDR for desktop is still very wip. They had two talks at XDC about what they had to do do to make it work with gamescope:

• The rainbow treasure map: advanced color management on Linux with AMD/Steam Deck - Melissa Wen
• Rainbow Frogs: HDR + Color Management in Gamescope/SteamOS - Joshua Ashton

Kwin has basic support for displaying HDR games through gamescope in KDE Plasma 6