Hello there,
My name's Stephen, though I've been posting under the "Karro" handle for years now, so why change now? (You can find me at RPG.net under the same name, but I haven't gamed in years now, and consequently haven't posted there in years either.)
I happened across this place while searching for GIMP tutorials to help me learn the program while I work on a fantasy world map. I've been drawing fantasy maps since I was a wee lad, and I'd done a number of maps of a particular imagined world (primarily paper-based). A couple years ago I decided to start recreating this world from the ground up, and using some of the maps I'd drawn when I was younger as rough guides. Using a free pdf I found on world-building I tried to learn a little about plate tectonics and how they form the world and affect the shape of the world above them. So, I sketched up a quick design of interlocking plates, gave a rough indication of the transforming, subducting, and diverging plates, and then placed my continents on top of them.
I decided that I wanted to be able to produce several kinds of maps of the same world without having to redraw the world each time, so I decided to try to work on the computer where, having in the past used Paint Shop Pro and having been introduced to the concept of Layers, I thought I could best perform this task. So, recently, I started redrawing the rough designs I did on paper in GIMP, starting with a basic outline of the tectonic plates, and then adding layers for some general continent outlines, and the ocean. I was manually drawing and blurring the shorelines of these otherwise featureless continents (I don't have a wacom tablet or anything similar, and it was taking forever, and I hadn't even tried forests or mountains yet) when I thought "There must be a better way... I'm not taking full advantage of GIMP's tools". So... after fruitlessly messing with some GIMP filters (like edge detect, and a cool polar map feature that I can use later to produce polar-oriented maps if I so desire), I decided to hit up Ye Olde Internet for better answers. It directed me here (unless I want to spend too much time watching video tuts... but I like static tutorial presentations better, so I can more easily refer back to them).
So, I already started fooling around with RobA's tutorial on regional fantasy maps, and I'm trying to think of ways some of the same techniques might be applied to a world-map scale. It looks like this is an excellent community, here, and I'm glad to have found it.