Why do you feel that GSX needs to be adjusted ?
The way GSX works is that it reads the Tropicalsim/DCA afcad file that comes with the airport. This afcad provides a layout and all other details based on which GSX plans its actions.
If things go wrong my first thought would be that the Afcad therefore has mistakes. You can use ADE from http://www.airportdesigneditor.co.uk/
to check the airport afcad and make corrections.
Also GSX has options to makes changes to the afcad as explained in the manual. When arriving at the gate and using option 6 you are offered various options. Make sure the pushback direction is not set to none in this case.