I can't walk you through it step by step because I know very little about Gimp but the process should be pretty similar to PS. Download a brush set, unzip it, place the brush files in the proper folder (probably something called "brushes" in the Gimp folder), and then start up the program. Once there, choose your new brush and check the brush properties then increase the spacing.
If the radiance of a thousand suns was to burst at once into the sky, that would be like the splendor of the Mighty One...I am become Death, the Shatterer of worlds.
-J. Robert Oppenheimer (father of the atom bomb) alluding to The Bhagavad Gita (Chapter 11, Verse 32)
My Maps ~ My Brushes ~ My Tutorials ~ My Challenge Maps
Currently using Photoshop CS3, GIMP, MapTool, and planning to try Inkscape, Illustrator and Wilbur "real soon now."
Running and playing as much 4e as I can.
All work I post here and refer to as "mine" can be used under this Creative Commons license