GT5 is perfectly fine with the DS3 controller, that's how I played Gran Turismo for the past 12 years. However, if you find yourself a die-hard racing fan then I highly suggest you get a wheel like the Logitech G27.
If you just going to be a casual player then there is no need to spend the money for a wheel.
But for me there is no other way to play Gran Turismo.
