I am still in the GIMP apprenticeship setting where I try to do per software what I do correctly per hand. I generally can't do as well with GIMP as per hand and sometimes I do outright worse.
But as I now know my GIMP to at least 60%, I think that I know where my biggest problem is.
It is in color transitions.

For instance there is a green forest that gradually thins out and goes over in a red/yellow desert.
I can do a correct forest and a correct desert but what I can't get is the correct transition.

Either there is a sharp green/yellow boundary that hits like a fist on an eye or a blurred zone that makes a color transition but destroys the fractal look of the map.
I also thought of making a color gradient layer superposed on a bump map which keeps the fractal features. It's almost what I want but the change is too regular.

What tool/technics should I use in GIMP to manage a color transition without destroying the fractal look ?