You can open desktop mode inside of gamemode, just like any other game or application. I find it easier to use most desktop apps this way instead of adding them individually to steam.

To do this, download this script. Save it as a script and make the script executable. Finally right click the file and “add to steam”.

Some important notes with this: You can’t exit steam while running the desktop this way, so apps that require closing steam (like steam rom manager) won’t work. There are also a few apps that do weird things, like cryoutilities shows up super tiny this way. But generally most everything works.