Assuming the white background skews in the same way a good trick is to make a copy of the image, blur it VERY widely, and divide the original image by the blurr

Assuming the white background skews in the same way a good trick is to make a copy of the image, blur it VERY widely, and divide the original image by the blurred version. This essentially removes any low frequency color/brightness shifts. I'll often use it to get rid of shadows etc on photographed paper, but it should work for a color gradient all the same.