Saving different maps into different mapset files also allows you to easily replace the map to newer versions in the future. However, if you have a lot of maps that you want to load into your GPS, or you want to store all the maps into a SD card which supports only a single gmapsupp.img, you might need to combine (join) the maps into one mapset file. GMapTool is a tool which allows you to join and/or split maps into mapset files.