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.
With the recent updates in stable, the reverse is also possible, you can run the gaming mode (big picture mode) in desktop mode. Not all features will work, but you do get most of the convenience of gaming mode navigation, with the capabilities of the desktop mode in the background.
My understanding is that game mode and big picture more aren’t the same thing. Game mode is running Wayland and Gamescope, allowing you to get much better performance compared to desktop mode, which is still using Xorg with KDE.
Correct, proper game mode is a Wayland session running steam big picture mode with exclusive special features baked in. That’s why I said you don’t get all the features, but you do get a mostly similar UI experience.