On Windows, the .gmapi appears as a folder.
On the Mac, the .gmapi appears as a file and not a folder. If you double-click the .gmapi on the Mac, it launches Garmin Mapmanager and asks if you want to install the map. So on the Mac, the "installer" is already resident on the system and is associated with .gmapi files.
So, my thought is to have a .zip archive containing two things:
1. The .gmapi
2. The NSIS .exe file
To install on Windows, double-click the NSIS .exe file, which reads its data from the .gmapi
To install on the Mac, double-click the .gmapi file, which is self-installing
Only one copy of the map data is needed to install on both operating system.