It was mentioned in one of their previous interviews as reason to parner with gog. I’ve unfortunately not bookmarked it, though, and I’m too lazy to search for it, sry.
edit: found it: “At the very least, GOG is sure that its support for Fallout: London won’t upset Bethesda,“They’re also our partners so we wouldn’t want to do anything to harm our relationship.””
Fallout: London Official Release Trailer (youtu.be)
Fallout: London | Trailer (youtu.be) angielski