I'm not sure about that scenery but, GSX removes all default FSX ground vehicles if you select a parking but, if the scenery comes with vehicles that are not standard FSX, they can't be removed.
Another possible issues, even if they are default vehicles, if you use time acceleration at 8x or more, then you go back to 4x or less, default vehicles that were removed will be restored. We might add an additional detection for this.