Well, their is more than one factor at work when a car pulls it;s front wheels off the ground. The first one is weight transfer, which we get in GT. The forces of enertia attemting to keep the vehicle at rest cause weight to shift off the front wheels and onto the back ones, uncompressing the front and compressing the rear suspension. However, to actualy pull the wheels up off the ground you also have to have the rotational force of the tiers. As they attempt to spin faster under acceleration the force of enertia is pulling back on the car. Now, if the tiers have enough grip, the engine has enough power, and weight transfer has made the front end light enough, then that rotational force at the rear axle will lift the front of the car off the ground. That rotational force is what GT dosent seem to model.