When you turn the beacon on, the Ground Power Unit automatically disconnects. It's an option in the EFB that you can turn off.
It's on the EFB options page, select "SIMULATION" in the left hand list, and the option is called "AUTO GPU DISCO" - set it to 'NO' and now the ground power should not disconnect when you turn on the beacon, however it appears bugged at the moment, and disconnects regardless of setting...