I guess it's the problem explained at Page 23 of the GSX Manual when discussing the parking customization features:
Pushback Direction
GSX reads the Pushback preference from the AFCAD file, which can be Left, Right, Both or None. Normally GSX will not allow pushback from a parking position that has its preference set to None, on the assumption the scenery designer had a specific reason to disallow Pushback on that parkign spot. But, in case the AFCAD is simply not accurate, this option will allow you to instruct GSX to override the Pushback preference set in the AFCAD with the one specified here.
