Was stuck at 92%. GOG Galaxy 2.0
1 Close the client (don't pause the update)
2 Open Task Manager > Processes > close anything related to client ( "GalacyCommunitacionService" for me)
3 Start client, and wait for it to finish > starting from 89% for me.
Update completed in under 5minutes after restarting the client.
EDIT: After restarting the Client / PC, game seems updated & working. 1.04