A GMM file is an XML-formatted text file. It is used to create .BMG files. These are binary files containing dialog and text from Nintendo games. Gamers rarely see GMM files. But they can open and edit them in any text editor.
Nintendo games have many proprietary file formats. Developers use Message Editor to create files with game dialog. GMM is one such format. It is used for compressed media like videos. GMT Player software supports GMM files. But it seems no longer available from the manufacturer.
The GMM extension indicates the file type. Windows apps use it to open the file. Different apps may use GMM files differently. We know GMT Player originally handled these files. But other programs may support GMM too. When Windows doesn’t recognize the extension, you must select an app to open the file.
Garmin MapManager
Garmin BaseCamp
GPSBabel