Thanks Freodin and Johnvanvliet.

Indeed you added both to the Learning curve .
Don't forget that I have only spent about 2 months on GIMP with a few hours here and there per month.
That's why when I do a map like this one, it is not to do a map but to try out a maximum of different buttons I have never clicked on and to understand what they do (for example I have already tested multiply but not yet divide)
Of course I am aware that there are surely megatons of forums and sites where hundreds of buttons and tricks are explained but I have simply not yet spent enough time to find them .

That's the advantage of this board - one can get concentrated knowledge coming from people who have 100s of hours spent with the software.