Oh no, not GIMP. Try SAI Paint Tool. I <3 it.
The blobs of light/dark seem very arbitrary, I recommend gnashing them into order once you get it into a program you're comfortable working in.
Unfortunately I'm broke. I'm just using the trial. Seems broken, but in a good way, as in the pop-up says it expired (apparently I've tried this thing out before, probably back in the day before I had a tablet) but it's running fine, other than no save feature.
Oh, on that note, here's a 30-minute-or-so doodle of some clouds (or just a very messy gradient), just getting a better grasp on the tools. Mostly "paper" brush here.