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. It can be crushed when driven, giving you a Machine Gun.
The name, GT-A1, is a play on words, meaning GTA 1.