This is a tutorial I did recently on how to do fairly quick and easy cell type character shading in Gimp. I tried to make it simple enough a beginner could follow it, which is why it's so long. The actual process takes a couple of minutes to do the set up on, then you can get to shading.

http://wastelander7.deviantart.com/art/Quick-And-Dirty-Cell-Shading-In-Gimp-383945156