Terminal only. Though in theory you should be able to expose a port to access an X or Wayland session remotely to use a GUI, but I haven't tried this.
Hardware raytracing works even on newer Radeon cards. I played Control recently with raytracing on Linux and it works pretty well, though the average frame rate drops to around 40 FPS. I had to use FSR to get higher framerates.
If supports xinput or directinput then it should work.In other words, if it works on Windows it will work with the Steam Deck.
This is some /r/surrealmemes shit right here.