I didn't use GIMP. Instead, I used a combination of Wilbur and PSP (an older version) to generate my maps for the latest Lite Challenge (can be seen here : http://www.cartographersguild.com/showthread.php?t=6823 ). I haven't written a tutorial yet, but the images shown were rendered by Terragen (version 0.9.43) with a little tweaking of the Surface Map and using SOPack. I'll be working on a "tutorial" shortly, which is to say, as soon as I get some time, perhaps this weekend.

GW