Agree with the above posts, Gamma is really only useful for wheels with low rotation 270 degrees etc, and speed sensitivity is really a kind of speed dependent Gamma.
I thing it's worth reading up on this excellent guide to AC settings,
http://simhqmotorsports.com/assetto-corsa-hints-tips-part-1/ from SimHQ Motorsports.
Gamma I think is just linearity. If you have high gamma it means your steering lock changes a lot as you steer. The steering lock itself can be anything. If someone says only 1:1 stering ratio is linear he has no idea what he is talking about. That's not how it works.
So in other words if you have a 12:1 steering ratio that is linear and you turn the wheel 12 degrees the front wheels of the car turn 1 degree. At 24 degrees the fronts turn 2, at 90 degrees the fronts turn 7,5 and at 240 degrees the fronts turn 20 degrees.
But if you have unlinear steering you may get 1 degree at 12, 24 at 2, 12 at 90 and 35 at 240. In other words your steering ratio changes the more you steer. At low angles you get 1:12 steering ratio but at 90 degrees you get 1:7,5 and at 240 degrees you get 1:6,8. It is practically impossible to countersteer with that kind of settings.
Speed sensitivety is not speed dependant gamma but speed dependant sensitivety. Speed sensitivety does not change the linearity of the wheel. It just changes its sensitivety with speed. At low speeds you may get 1:8 steering ratio but at little higher speeds you get 1:12 for example. But no mater how much or how little you steer you still get just one steering ratio with speed sensitive steering setting.
If speed sensitivety was like speed dependant gamma then it would mean that your linearity changes with speed which doesn't happen. Linearity and sensitivety are two different things. Linear is always better but sensitivety is matter of preference, equipment and available options.Of course you can make your wheel less sensitive in the middle and more sensitive when you turn the wheel more with unlinearity but in 99% that is a bad idea imho.
And when talking about brakes fully linear is better as well. Any unlinearity is just direct reduction in pedal accuracy. Some games for some bizarre reason claim that fully linear brake pedals are only meant for loadcell brakes. But put it nicely that is hogwash.
