GMAP files exist as 2 different types. Please find information about each of them below.
Type 1: Garmin Map File
GMAP files are binary. Their details are not public. Files used on Garmin devices: GPS Exchange Format (gpx), Fitness file format (fit), Garmin GPS Database (gdb), Garmin Training Center (tcx), GPS Lo...
GMAP files are binary. Their details are not public. Files used on Garmin devices: GPS Exchange Format (gpx), Fitness file format (fit), Garmin GPS Database (gdb), Garmin Training Center (tcx), GPS Location (loc), Custom Map (kml or kmz).
BaseCamp software manages GMAP files. GMAPs are compatible with Mac and Windows. The GIS Files subset has 255 formats. BaseCamp was made by Garmin Ltd. More GMAP or BaseCamp data is on their site.
Online apps allow GMAP actions without installs: view, edit, convert, merge, split, compare. Click links to see features.
Opening GMAPs needs compatible software. Garmin BaseCamp works on PCs and Macs. To open in BaseCamp: Launch it. Go to “File” and “Open.” Find the GMAP file and click “Open.” The GMAP appears in BaseCamp. Garmin GPS devices also read GMAPs directly.
GMAPs relate to the Garmin format for devices and apps. They seem not convertible to other formats. This is common for system, temporary or proprietary files. It protects developer IP.
GMAPs are scarcely in user files. They work on Mac, Windows, iOS. They open wrongly or have no program assigned. To solve: Double-click the GMAP. See if your system assigns it correctly. Or manually select the software for it.
The GMAP file type is GIS. Garmin made it. It works on Mac and Windows only. Alternatively, use BaseCamp. GMAPs open wrongly because they associate incorrectly. To solve: Double-click the GMAP. See if it opens properly. If not, manually select software for it.
Garmin BaseCamp
Garmin MapSource
Google My Maps