This issue can be solved by clean reinstall, please follow instruction below to do that:
Please remove the GOG GALAXY Client app from the Applications directory.
Please also remove the contents of these directories:
You can open it using the "Go to Folder" command accessible with the Command+Shift+G key combination or selecting Go > Go to Folder from the Finder menu.
Now restart your Mac, download the current installer from:
and reinstall the GOG GALAXY Client.
Don't forget to import your games back after following above, instruction how to that can be found here.