Noishkel: Ok... now I've found a fix to prevent Galaxy from auto updating.
Download the 2.0.71.2 client, uninstall your current version and then install the older version. But don't let it run.
Then navigate to C:\ProgramData\GOG.com\Galaxy\redists, then rename the Galaxyupdater.exe to something else.
Run Galaxy and do a 'scan & import folders' command from the 'add games & friends' menu. That'll reconnect your installed games to the Galaxy client. You'll probably had to let it install some small updates to complete the resync
This seems to be working for me and it should be a temporary fix until GOG can un-f*ck the Galaxy client.
Thank you very, very, very much! This solution works like a charm. finally I can download my games again :)