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

[Ofnuts]

Try this:

• Image>Mode>Indexed and let Gimp create the optimal color map
  
• Windows>Dockable dialog>Palettes. The top palette should be "Colormap of image...."
  
• Right-click, and Palette to gradient
  

You can then Ctr-Z to restore your image to full RGB. Experts will wonder about the stint via Color-Indexed mode ince one can create the palette directly (Import Palette>From image) but in that case the palette in not ordered by luminosity.