Economy Garages uses these two terms to refer to the orientation of the trusses in reference to the garage door.  A building with a gable orientation is one in which the garage door is located on the side of the building with the truss peak; whereas a building referred to as reverse gable, has the garage door on the side in which the slope of the roof is visible.  The reverse gable designation helps indicate that a much higher load will be placed on the garage door header which will require a load calculation to ensure the proper header size is used.