I used a sidewinder joystick (nothing like force-feedback or anything, just a bog-standard one) for about 5 months in GPL, before getting my wheel.
I went negative with the joystick, to around -25 as I remember, but the 1st day I got my wheel, I was matching my PBs. I'd never had a wheel up until that point, and hadn't even driven a car in real life I don't think, a tractor being the closest thing to it
I'm currently on -60ish, and if I'd still been using the joystick, I doubt I'd have cracked -35.
To put that into some form of perspective for those not knowing GPLRank, there's 11 tracks in a GPLRank, all them defaults that come with the game, and your GPLRank is calculated by adding all your fastest times in any car together, then subtracting it from the total of the demo laps which come with the game. You've 'gone negative' if your total time is less than the benchmark demo times. In my case I'm 60 seconds faster than these benchmarks, or 30 seconds faster than if I was using my joystick.
Remember gamepads generally have less movement to play with than joysticks too, so are usually a lot more sensetive. Fine for Toca Race Driver and other arcade games, but totally impracticle for simulations, unless your name is Stuart Knibbs.