GOG does offer a wrapper for steam API, so in theory there is no requirement for a separate branch for making a build.

Though there's also no requirement for approval of a patch...
And I have seen Devs claiming it was very annoying to push patches to GOG because of the manual verification process for patches.

Your comment made me think of all the crap quality games and how having an instant update could be a vector for malware. But a manual verification for offline updates, would act as a protection layer. As well as dissuade a would attempt, knowing someone has to provide approval.