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

[Dave Rove]

How do I create a gradient map from an image?

If I have an image that I know started off as greyscale and that somebody has colourized it with a gradient map, and that the resultant image is sufficiently detailed to have all 256 lightness values mapped to a colour -- then how do I create a colour gradient map from that image so that I can colourize images similarly myself?

I know that this is possible because I did it myself a few years back and I have the .ggr gradient maps that resulted, but Google has failed me now and I've forgotten how I did it.