it just takes practice to drift with the G25/27 on GT5. and alot of throttle control.
the easiest way to learn how to drift in the game using the wheel is just running a normal race with whatever car you want to drift. or run practice mode if you dont want other cars around.
run the car you want to run, no traction control, ABS is up to you. and start to use a bit more power on the exit of the turns, i'm not talking about stabbing it to 100% throttle straight away. but a bit more than what you usually would. then once your comfortable with it, start putting more power on at half way through the turn, then at the start and finally before the turn even starts.