How do I create a gradient map from an image? [solved]

[dinasset]

Did someone try to:
- desaturate the image
- use the gradient so created to rebuild the image in colours
I made some attempts: no success (outcome maybe nice from a crazy painting viewpoint...)
If someone succeeded please:
- post the original (or the one reduced to 256 colors)
- post the gradient created
- post the desaturated
- post the rebuilt
Just to learn...