The GT-A1 is a vehicle in Grand Theft Auto 2 that can't be found at any of the districts of Anywhere City in a non-multiplayer game. It can be seen, since the Wang Cars are GT-A1s with numbers on the top, but can't be driven (entering in one of them teleports you to the Wang Cars site).
It is possible to drive the GT-A1 with the debug keys cheat. To do this, find a GT-A1 and press 0 on the number pad repeatedly a moment before you enter the vehicle. If done correctly, you exit the vehicle after you enter it. Then you can reenter the vehicle and drive it.
The name, GT-A1, is a play on words, meaning GTA 1.