A GMAP file is a GIS file that contains a combination of specific regions of the world. It was used by some Garmin devices. The map combinations can comprise of different regions that need not necessarily be contiguous. Garmin devices used to store these map files in the embedded memory. However, newer Garmin devices support mass storage mode that appears similar to a normal USB storage device. GMAP files can be opened with Garmin BaseCamp and MapInstall software on PC and Mac software.
GMAP File Format - More Information
GMAP files are saved to disc as binary files and their internal file format details are not available publicly.