I have to add that, another reason why the Pushback doesn't start, is the AFCAD has set the Pushback preference to "None", and GSX obviously respects that, because there might be real world constrains and reasons why one scenery designer didn't want Pushback on a scenery (if there was "no preference" between left or right, the correct choice should be "Both" ), and GSX can't obviously decide on its own if a parking is set in a specific way intentionally, or by mistake.
Of course, using the "Customize parking position" option in GSX, you can override the Pushback preference of the AFCAD, without having to edit the scenery AFCAD.