Comment by jakkos
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.
The problem was not only in KDE but also in NVIDIA drivers iiuc. For my setting HDR has been stably working on KDE since early 2025.