Sorry for my late reply but let me clear up why I've been wondering about this.
Steam / Origin / ... are all closed source because they want to control the client side, they don't want people to be able to strip their DRM / tracking / ... out of their client.
GOG is a more open platform where they present Galaxy as optional, therefore I don't see the problem with people forking Galaxy, since it is just a front end for installing games. They don't need to control Galaxy since it doesn't make them any money from tracking their users (I hope.)
bram1253: I have been pondering this question during the evening and I can't find a reason why this shouldn't be the case, therefore I'd love to know your thoughts on the matter.
Cambrey: Newbie question: what would one do with an open source Galaxy? Just curious...
It would allow programmers to make changes to Galaxy as they see fit. They could, for example, add custom themes, improve speed, port Galaxy to different platforms, add more options like being able to download all your games as a backup, ...