As Fallout 4's large Fallout: London mod nears its launch, distributor GOG has moved to clear up some compatibility …
Source : https://www.eurogamer.net/fallout-london-mod-will-work-with-fallout-4-on-gog-and-steam-but-not-epic
Fallout: London mod will work with Fallout 4 on GOG and Steam but not Epic – Eurogamer
