What do you mean by "changed departure airport", exactly? Going back to the Main menu, select a new departure airport and a new gate from the World map?
If that's exactly what you did, GSX is not involved in this: when you click on a gate on the Map View, it's the simulator itself that is repositioning the airplane, not GSX. GSX enters into play only if you choose "Reposition" from GSX menu, and select another gate or the same.