Truehare: I've been having this problem for weeks, and this solution worked. I can confirm that you need to disconnect the Steam plugin and shut down GOG Galaxy before deleting the folder (tried it without doing that and it didn't work).
So, to reiterate:
- disconnect the Steam integration plugin
- close the Galaxy client
- delete the folder mentioned by LordChen
- restart Galaxy
- connect the Steam integration plugin
- voilà, problem solved.
Oh, and I can also confirm your last paragraph. GOG is really one of the best things to happen in gaming since forever. They make some mistakes along the way, but there's no ignoring their core philosophy of always trying to do right by us, the customers. Here's hoping that philosophy never changes. And they're not even paying me to say any of that! :D
Veritas.Deus: Doesn't work for me. Still "Offline - retry".
EDIT:
I managed to get it working. Steam Guard integration was the cause.
So if you have Steam Guard set, you have to delete steam plugin from two folders:
%localappdata%\GOG.com\Galaxy\plugins\installed\
%programdata%\GOG.com\Galaxy\storage\plugins\
After doing the above, and what was said before, it works.