First of all, you should use a 16bit file, if you're not already doing so. If you want the exact end colors, then there's not much you can do, as there are only so many step the gradation can take.You can change the end points, so there is not such a wide difference between the two (Lighter blue, or bluer white).
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Okay, I'm trying again with a video response! Assuming the change to the gradient is to reduce the contrast of the back "wall" and not the ground plane, Here's a video explanation of one of many ways that this could be accomplished.
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