In the GIMP 2.10.34 one can extract an RGB image's Hue values as a grayscale map where one might assume that 0-255 represents 0-360 degs of hue. To my dismay, the grayscale values are not at all linear with respect to hue degrees, something to do with GEGL according to ChatGPT. So it is that green (120 degs) does not show up with a grayscale value of 85/255, instead more like 10/255.
For anyone interested in a more correct grayscale hue map here is a free on-line app that gets it mo' right: No doubt there are others ...