You should first check if GSX is using the 3rd party scenery AFCAD. This can be easily done by selecting the "Customize this parking position" option, the dialog will indicate the AFCAD in use. If the name is something like APXxxxxx.BGL, it means the default AFCAD it's in use.
If the scenery AFCAD has problems that can't be fixed automatically, like wrong links, orphaned parkings, unreachable parkings, etc, it might be rejected by GSX so the default will be used instead. If the scenery AFCAD is without faults, GSX will use it without any special configuration.