Script-Fu -> Artist -> Angled Strokes
This is a non-adjustable script which renders the original image with a diagonal brushstroke. The more contrasting areas you have, the more stroking that goes on. Can look quite the mess on images with gradients, but if the original has the right amount of contrast and bold edges, the effect can be very satisfying.
Not the most effective use of the script-fu below but it does give a decent idea of how the script works on gradients and edges.