Here is a picture of the way that I heat the GPU and the CPU. I use two Xbox 360 cases with the top wrapped in tin foil (to keep it from melting the plastic of course). I position it so that I have a clear shot of the complete underside of the GPU and the CPU. The weight on top of the GPU and the CPU is four nuts that I have wrapped in tin foil (again the reason for the tin foil is that I am trying to keep as much heat off the top of the board as possible) The nuts add up to be 4.4 ounces. I have put more weight before but it is too much as this seems to be perfect.
Here is a picture of my setup. As far as the distance from the chip, I have the heatgun about one inch from the chips as I heat.

Woah, the heatgun is 1 inch from the cpu/gpu, this seem a bit extreme. How long do you keep the gun there? thanks