On Gimptalk someone was asking about the "Gradient Map" function. Asking what the Gradient Map function really does and how it works. This response was taken right from Gimptalk, which was taken from Gimp help. I didn't write the answer, I only created the test images.
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. (...) 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.
I created this test image. Feel free to download it and experiment.
Gradient Map Demo.jpg [ 45.81 KiB | Viewed 352 times ]
Applying the Gradient Map to the test image, shows no effect with the default black and white. If you reverse the black and white source colors, Gradient Map makes it look inverted. Running it with a color gradient, shows what it's really doing. Here is an example of the effect, using the "French Flag" gradient. Trying it yourself is the best way to see it. I hope this makes it easily understandable.