You can think of this command as a “wrapper”. You need to put this in the advanced properties launch options of every steam game you wanna play.
If you want something easing, you can create the folder ~/.local/bin to put custom commands in, and make the following file at ~/.local/bin/gamescope-wrapper
Then you would do chmod 755 ~/.local/bin/gamescope-wrapper to make the file executable. Then, you need to add the directory ~/.local/bin to your $PATH. This is so you can type gamescope-wrapper instead of ~/.local/bin/gamescope-wrapper every time you want to use this script.
Then, you go to the game you wanna run this with, go to the advance properties menu, and go to the launch commands. In the box, you’d type gamescope-wrapper %command%