File created by Gameboy Map Builder (GBMB), an application that allows users to design maps used in Gameboy games; contains map information such as the width and height of the map in tiles and number of property colors; includes the path to the .GBR tileset file that is used to create the map.

To create a GBM file, you need to first select a tileset. Select File → Map properties..., select the size, choose your GBR tileset file, and click OK. You can then design your map, select File → Save or Save As..., name the file, choose the save location, and click Save.