'SetSteerGyroscope()': simply keeps the wheels center to the car when no steering input is entered, it doesnt actively counter oversteer
Not exclusively. With the gyroscope disabled, the wheels still auto center when there is zero input, I can provide video of this if you would like. With or without the gyroscope, after light turning, the wheels seemingly appear to counter steer ever so slightly, even if the angle is only slight and there is no discernible change to vehicle direction.
and stabilitygrip change sounds like its simulating the grip increase of spinning wheels slowing to match relative roadspeed.
Not really, a slow down in wheel speed that would result from the tires biting/finding traction/grip is not a manifestation of the tires being directly given extra grip above the amount of grip generated by that tire. It is worth noting, that the 'StabilityGripChange()' changes grip, and doesn't affect traction, so theoretically it has minimal or even zero affect on a car that is burning out, instead it affects turning & sliding only.