The gain in speed coming from the elevation (ignoring loss due to friction) is equal to the change in potential energy of elevation, which is mgh. The work done to reduce this to a zero gain in speed is equal to the work you would do going up that hill. If you are going down that hill, the energy that would (with no speed control) be added to your velocity is equal to the work you would have to do to go up that hill. If you are going down that hill at 20 mph, how quickly you would have to work to account for all that energy is exactly how quickly you would have to burn energy if your were pumping that bike up the hill at 20 mph. If you can pump your bike up that hill and maintain a 20 mph, congratulations; you can work hard enough to counteract all that speed you would otherwise be gaining as you go down the hill.

I'll let TE tell you how many horsepower it would take to go up a 20 degree slope at 20 mph (or how many brake horsepower it would take to prevent any speed gain going down a 20 degree slope at 20 mph).