Bohan is the best place I have found. I found a regular Futo there one night, and drove it around for 4 hours, no GT. I thought at that point it was a hoax.
Try 2, took 2+ hours even to find a regular Futo, finally found one while driving a Turismo through Bohan. Once in the Futo again, I spotted the GT within minutes, was just watching for the gray vented hood. The best route I found for Futo spawning was the major road from the bridge from Algonquin over to the strip club.
Please note that the GT is just cosmetic changes, not actually a different car. Not sure if it was worth the 7 or so hours I spent looking for it. Nonetheless, I have mine painted "panda" style a la Initial D (it was indeed red when I got it) and it's going to be in my parking spot forever. Fun little car.
