If GSX presents you with a list of parkings, it indicates it doesn't detect you as being "in" a parking. This usually indicates an AFCAD conflict, so the wrong AFCAD is being used.
You can check which AFCAD is used by GSX, selecting the "customize airport positions" from the GSX main menu, the name of the AFCAD in use is listed in the top left. It should be the one that comes with the scenery.