No registry data is needed for the gmap format. Just copy the folder to the correct directory and the map will appear on the dropdown. An author might choose to use the registry to help with things like removing the map, but it's completely optional. And I think that's the point, since gmap was first developed on the Mac which (thankfully) doesn't have an equivalent to the Windows registry.
True, but does "the correct directory" mean "any directory"? If the directory is a specific one (don't have a Mac, but say "Garmin\ Maps") then the application would only need to look in a limited number of places to find map products, which I believe need to be "Installed" on both Macs and PCs. "Garmin CustomMaps" however are "imported" like other user data. Too bad they are called "Maps", because that's the source of confusion.
The Mac version of BaseCamp has an option to switch off overlays (and/or text labels), but I can't find such an option in the Windows version. Guess it isn't possible there...