"GOG GALAXY is a fully optional client to install, play and update your games.
It also offers online multiplayer, achievements, chat, game-time tracking and
more - but it’s up to you which features you want to use."

"GOG Galaxy requires at least Windows 7 or Mac OS X 10.8 and soon is coming to
Linux."

Like Steam, even for the Windows version,
the downloaded files would come up hopefully in a predictable location GDP can
repack from.