The original is on the top half, and the darker version on the bottom.
The gradient file itself is a 256x128 dds file that I just opened in photoshop and went to image>adjustments>brightness/contrast and played around with that.
What you see above was my first attempt. If I push it much darker, the image in the game will become totally flat from lack of contrast.
Edit...
I think the left of the image is dark, and the right side is bright lighting... I'm going to darken the left side much more and see how that looks.