GSX can only use information which exists in the AFCAD. If the parking is flagged as a Left, but the only nodes that comes out of it in the AFCAD result in a pushback to the right, that's what GSX will do so, nothing you do in GSX can change that.
Or, it's possible that nodes might be available, but cannot be used because they were placed wrongly, overlapped, orphaned, too close inside the parking radius, so GSX ignored such nodes, because they would result in errors like the plane making a 180 deg while being pushed.
So, basically, whenever you see something "strange" in GSX, it's almost invariably a problem with the source AFCAD, but of course I cannot precisely tell what it is, without having a look at it.
You might also TRY the "Custom Pushback" option, which allows you to specify the destination node, but it still must be an existing node.