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

[Zero01]

You could use the G'MIC filter under Rendering>Gradient [from line], to get the colours you wish to use for colourizing purposes, save that (just give it a name in the 'Save gradient as' box at the bottom of the dialogue) then refresh your GIMP gradients tab, choose the gradient you created and then apply it to the image you would like to colourize by going to Colors>Map>Gradient Map.
There may be another way just using the GIMP core features but if there is, I have also forgotten it.