Only way I can think of doing this is with some really good skill with spray paint or an airbrush. I'd recommend getting a can of each color and doing a lot of practicing on pieces of cardboard or something.
Here's my idea: paint exaclty 1/2 of the case one color and 1/2 the other color. This way, when you do the whole fading thing, the lighter spray won't expose any of the origional color of the case. Then start with doing the fade painting over it.
Another suggestion would be to paint the entire case gray and then paint the silver portion of the gradient on top. actually, i think this idea would work better than the half and half idea.