Same here. I had nearly all the XCOM2 DLC purchased from GOG, and then Steam ran a sale on the bundle that was cheaper than buying the last piece to complete the collection! Since then I think GOG have run similarly cheap sales, but it wasn't the last time I saw that happen.

I know launchers like Heroic are available, and I use it for some of my games from them, but I actually liked the Galaxy launcher on Windows. I wasn't linking it to anything else though, so I didn't run into the issues you mention.

It's sad, because I think they could do well in the Linux community. Hopefully they eventually start supporting it, but until then I'll be buying most of my games from the company that's actively contributing and improving things for the community.