Forking an existing integration wouldn't make it ''official''. Yes GOG would be responsible with maintaining it, but it would still not become an officialially endorsed thing from Steam or others just because GOG is doing it.
When I say "official" I mean an officially supported feature of Galaxy 2.0. It doesn't have to be officially endorsed by Valve. It just needs to be something that GOG takes responsibility for, rather than just saying "oh that's a community feature, don't talk to us about it."
That doesn't make sense when it's literally the main settling point of this app.
I love GOG and I really want this to succeed but if you advertise a feature, you can't disavow support for it.