I just found a thread on Steam forums where a guy got files from the DLCs from the Mac version of the game (the Mac version comes with the DLCs as seen here: https://itunes.apple.com/en/app/grid/id594773178?mt=12
) and added them to the game's folder on Windows.
So if you get the files that the Mac version has and are not present in the Windows version (except for runnable binaries and libraries, of course) you should have access to the new content.
I haven't tested it (I actually don't have the game yet) but some people on the Steam forums claim it works.
Hope GOG releases the Mac version so that everyone here can try it.