Which is of course not the case. When mobile, you switch to off-line mode and use the maps that are stored on disk. The program started off with only support for the EarthMate. But now I have also added support for the TripMate, Garmin GPS units and the NMEA protocol, which allows you to use many others. Take your pick.
First step is to install the program.
In the program directory you will find an executable and some configuration files. It is a good idea to create on your desktop a shortcut to the executable. In case of the latter it is important you find out which COM-port is used, in my case COM6. GEM will load the above map and will then try to connect to your EarthMate. The device type and port-number will be saved, so you will have to do this only once.