Huh? I really can't be convinced of that. The force of the air hitting the top of the gurney flap is a force downward on the back of the car - very little, if any, differential pressure is causing any significant downward force. It's like saying that the shape of an airplane wing causes a "vacuum" of sorts on the upper surface, causing significant lift to make an airplane fly. Sticking your hand out the window of a car while it's moving, and "flying" your hand in the wind is evidence of that. The angle of incidence, relative to the wind, causes the wing to be pushed up from underneath.
Ed Nerobro wrote, You're putting a huge air dam under the plane. This is essentially the same as a gurney flap on the back of a stock car. While the flap itself isn't making downforce on the car, it's making the air under the wing, and quite a distance out to either side of the flap pack up, and provide greater differential pressure.
