Gradient to color levels

[rich2005]

Maybe something like this:. Depends on the starting point but you can use RGB

(1) Colors -> Posterize will reduce the number of RGB colours  or Greyscale shades

and

(2) If RGB then Colors -> Desaturate -> Desaturate reduces to grey & the image is still RGB

or  Image -> Mode -> Greyscale will give an 8 bit image.