You should be able to add it to your steam library as a non-steam game and then in game options in Steam set up using Proton as long as you have Proton enabled for all games, not just verified games.
@Dubious_Fart@Raincloud I get this error when I play some games from GOG through heroic which happens on steam deck. Also, controller support is not great on gog through heroic. So, this makes me stay on steam to play games. Anyway, I hear that steam has drm free games so if BG3 and Star field are drm free then it doesn’t matter if it’s on steam or gog.
Heroic supports Epic, GoG, and now Amazon games. Supports installs, updates, file validation, and uninstalls perfectly. I think for Epic it supports cloud saves as well, but I don't use that feature and have heard horror stories from early adopters of it.