If you want to save space on your system disk, it is sufficient to copy a Shortcut to the GMAP folder instead of the folder itself.
Sometimes (for me, at least) Basecamp does not see the map in Program Data but does see it in C:\Program Files (x86)\Garmin\BaseCamp\Maps\
Sometimes (for me, at least) Basecamp does not see the map in Program Data but does see it in C:\Program Files (x86)\Garmin\BaseCamp\Maps\