Heroic Games Launcher supports GoG pretty well. You can install it with Flatpak using Discover in desktop mode.
When you login to GoG and install a game, you will be able to “Add it to Steam” (top-right hamburger menu in game’s view) and it should then be available in your “Non-steam games” in gaming mode.
Other options are Bottles and Lutris
Yes, DO AS I SAY!