Why thank you ChickPea
I'm not so sure that being a champion of something means I'm necessarily the best, however. There are a lot of CC3 users here at the Guild, and some of them have drawn maps I just would have no idea how to start! LOL!
Gildor - I use both CC3 and GIMP. I tend to use them together these days - exporting from one to the other and back again.
If I want to do just a quick map I use CC3 because it takes a fraction of the time. If I want to be able to draw my own cliffs, gorges, mountains and trees one by one, or if I want to incorporate a large amount of art in the form of original hand drawn bitmap images that I can edit in situ I use GIMP.
I have one major observation in the comparison between the two. In my opinion CC3 might have something of a learning curve, but GIMP is definitely not the soft option.
EDIT:
Costlines in CC3 are instantly editable, and can be fractalised at the click of a button. Whereas in GIMP you are likely to just have to keep redrawing the coastline until you get it right. I might be wrong, but I've never heard of an automatic coastline tool in GIMP.
CC3 itself has several sets of ready made symbols - anything ranging from mountains, through trees to town and city icons, sea creatures and cartouches. The CC3 add-on City Builder 3 has automatic streets, automatic buildings with accurate roof shading, and I think it also has automatic cities (not sure on that last point because I prefer to paste my buildings individually).
The best thing you can do is plunge into the tutorials for GIMP and download the software to have a play, and also use the trial for CC3 to compare notes on the two for yourself - remembering that it is necessary to work through the manual example if you ever wish to be proficient with the software
Here are a couple of extracts from my CC3 maps. The first is an overland map drawn entirely from the built in symbols and fills, and the second comes from a top view dungeon style map - shown here to illustrate a very small selection from the vast collections of freely available 3rd party symbol and fill sets