There is no HDR protocol for Wayland, so Wayland compositors cannot display HDR content. X11 also does not have HDR support, so window managers can't do it either. Games don't draw their own windows. If the compositor doesn't support HDR, the game isn't gonna be displayed in HDR. If the compositor doesn't support VRR, then VRR is also not going to work. Plasma 6 implements a custom Wayland HDR protocol which is also implemented by the Steam Deck. This allows it to display HDR content. It has also had VRR support for a few years. GNOME is waiting for the official HDR protocol for Wayland to be complete before implementing support, so it doesn't have it. It also didn't have VRR until relatively recently.

If you don't believe this, then can you explain why Plasma devs have been advertising HDR support as one of Plasma 6's killer features? Can you explain the purpose of this recently merged VRR merge request for Mutter? What is the purpose of this merge request for a color management/HDR Wayland protocol which has been in the works for years? If compositors have no involvement in HDR or VRR, then why are their developers working their asses off to make it work? And most importantly, can you show me HDR on Linux working in GNOME? Or Cinnamon? Or anything that's not Plasma or Gamescope?