Reinstall Galaxy client as admin to path X:\Games\GOG\Galaxy\ or ...\GOGGalaxy\. Set Galaxy exe in that folder runasadmin in compatibility.
Reinstall game to similar path X:\Games\GOGgames\. Set all game exe's runasadmin (optional if all can be executed from Galaxy client that already started as admin from game MORE section).
When reinstalling all that remove folders in Program Files and in Win user AppData\VirtualStore\. Make sure that win registry no have entries abot Galaxy client (not GOG in general) and about installed via Galaxy games (Mafia and if any other installed and uninstalled with client - all of them too). After downloading of each game you must check game files in Galaxy. better twice.
Mafia Configuration Tool must started as admin and make entries to both HKLM and HKCU sections of Windows REGEDIT.
But PCGW, same as any wiki, possibly not 100% accurate.