HDR videos and games (both native and proton) work in both KDE and Gnome (and supposedly Sway and Hyprland, but I haven't tried either). I think support in KDE/Gnome landed in a stable release ~6 months ago.

The HDR experience on KDE is about as good as the Windows one. Last time I tried Gnome, there was no way to configure SDR and HDR brightness separately, but it was definitely still usable.