Go to Desktop Mode, run Plasma Discover, get the Heroic flatpak, run Heroic, log into GoG there, install games.

Heroic is pretty damn good at doing the rest. It’ll install the Linux or Windows version of your games, it’ll add them to Steam, it’ll run them. Heroic will even give Steam some coverart for your games. (Many are missing the logo, tho. DeckyLoader +SteamGridDB plugin fixes that.)