Example of gradient map
This filter uses the current gradient, as shown in the
Brush/Pattern/Gradient area of the Toolbox, to recolor the active
layer or selection of the image to which the filter is applied. To
use it, first choose a gradient from the Gradients Dialog.
Then select the part of the image you want to alter, and activate the
filter. The filter runs automatically, without
showing any dialog or requiring any further input. It uses image
color intensities (0 - 255), mapping the darkest pixels to
the left end color from the gradient, and the lightest pixels to
the right end color from the gradient. Intermediate values are set
to the corresponding intermediate colors.