I have downloaded only a few aircraft and none of them have the GPS screen.  Am I doing something wrong or don't all the fairly recent aircraft, not Sopwith, have them?  When I hit the GPS key I just get a few instruments. ???

The "shift" key  with any number (1-9 I believe) just opens and closes different windows. In most panels,  shift+2 is the radio-stack.

Any panel (radio-stack, compas, GPS, overhead, etc.) can be assigned to any of the windows. More times than not, the GPS is "3".

It all depends on how the panel was designed.