Actually, it was designed for the GP40 but the GP42 has the same mounting lugs so there will be no problem in mounting it. When I mounted the GP42, the trainer balanced perfectly level. I only had to move the battery pack a little to achieve a slight nose down attitude on my balance jig. I like to have trainers just a little nose heavy. This is one of the reasons that I say that the GP42 is a perfect match for the trainer. It has sufficient power to take off with authority, does loops at full throttle without tending to stall, and has sufficient power to get you out of trouble when those unexpected events occur.