the heroic launcher was recently updated to support gamescope (installed it through flatpak by installing org.freedesktop.Platform.VulkanLayer.gamescope 23.08 through the terminal using >flatpak install).
it can be configured with a simple GUI (as seen above) and gives you all the options you could reasonably need. One of the big improvements is that on wayland, unlike when used through steam, it will actually close the game when told to, without requiring you to maunally kill the gamescope process.
valve still requires you to edit text launch options to enable and configure gamescope.
Valve needs to do better with gamescope, this 3rd party FOSS app is embarrasing them.
Yeah. I don't know what those flags are... Yet.
The moment a game launch fails in that particular way, I will be diving all the way into that particular deep end.