I've always appreciated gog releases. I have an account there and installing their games on the steam deck is pretty straight forward from desktop mode.
If given the option, use a good old games released game instead of a steamrip. It'll be better in the long run.
While usually true, this isn’t always the case. Some games have differences in code between the versions released. Modders especially will butt up against this from time to time.
If I installed a GOG games to my linux desktop, can I just copy the install folder to my steamdeck or will I need to run the installer again on the steamdeck?
So far as I know, it would be a proton instance. If you copied that whole thing over you should be fine.
Then just add to steam and tell it to use compatibility.