Yes, GAP insurance would only be of use if your vehicle was stolen or otherwise found to be a total loss and your primary insurance policy paid out actual cash value for the vehicle. You cannot make use of your GAP insurance policy if you are upside-down on your loan and wish to sell the vehicle.

Gap insurance is intended to pay the difference between a covered insurance loss and the balance of your loan or lease. If you sell a vehicle, this is not an insurance loss and thus a claim with your Gap insurer could not be placed.