At the bottom of the Position/Time page you can insert the time differnce between UTC and local time (which you can figure out by clicking at the cloud icon in the sim's top screen menu). This will then then display the local time on the Position/Time page of the FMS.
I guess in the real world, the GPU has to be removed before the beacon is set to on, since this shows the plane is ready for pushback. And a pushback with the GPU connected is not possible.