With this script is easy to create gradients that they simulate a sunset or a natural landscape. You take a photo panoramic, it selects a thin vertical strip and it rotates it in counter-clockwise direction. You read the maximum width of the image and insert it (width / 2) like number of segments in the dialog box of the plugin.
On the left original photo, to right the gradient.
Author web site: xoomer.virgilio.it/lwcon/gimp/python/make-gradient.htm