Successfully complete driving school with at least all bronze awards to unlock the Super GT. Alternately, in San Fierro, go to the Easter Bay Airport. There will be a small building next to the entrance near the helipad area. Below that building, you may find an orange Super GT. It does not usually appear as soon as CJ goes there, but should spawn after awhile. It may take some time for it to spawn. It will be locked, but you can open it by towing it to your garage with a tow truck.