Back in the days when I learned how to make patterns, there was a graphics program called Paint Shop Pro by JASC. It has been subsumed by Corel since those days and replaced by Paint Shop Pro X5. Nevertheless, the program was remarkably similar to the Gimp. An American scroller called Andy Deane wrote a tutorial on how to use PSP to create shadow portraits, and that was how I learned. I see his tutorial is still available here and it would be very easy to adapt it for the latest incarnation of the Gimp. Obviously, PSPs tools are laid out differently from the Gimp so you might have to hunt around for them, but essentially they're very similar.
Anyway, I reckon aspiring pattern designers could do worse than take a peek at Andy's tutorial.
Anyway, I reckon aspiring pattern designers could do worse than take a peek at Andy's tutorial.