This is my first attempt at making something in Gimp, I basically laid down a clouded terrain with a brownish red hue using turbulence to make the land appear cracked. I used the mountains tutorial by Roba to create mountains, although I did mess up a little bit and it made some weird circular lines around them that actually ended up looking kind of cool with blending. Overall I still have no idea what I am doing, but I thought this came out nicely. I also ran the cartoonist feature to bring out some relief lines to create a sense of depth.
As a first creation, this isn't bad at all. You have the right ideas. Your efforts to create a sense of depth work in some ways (and not so much in others, but hey, it's your first time!) Now it's a matter of technique. I recommend posting this in the "Regional Maps" forum as a work in progress.