Easiest way is to use heroic. You can easily install it from the discover store, log in with your gog account and click install. Then right click and add to steam directly from the heroic interface, this way it will show up in the deck game mode.
Ps: I know you wrote you don't need a fancy launcher, but this is legitimately the absolute easiest way to go about it.
has it been a nightmare? I'm genuinely asking because I have it set up (with nextcloud master container) and although I don't use it very often, the times I do it seems quite alright