What works for me is setting two gravity values, one maximum and one current. The maximum is how strong gravity is at all times, and the maximum speed you fall with. Current gravity increases from 0 to max the more you fall, at your specified rate, but also when you jump you set the current gravity to a negative value so that you fall upward for a bit, the specified rate keeps adding to it until it reaches 0, and then keeps increasing and you start falling again.
Hope this made sense and that it helps