You're in a position a lot like I am. I'm working on my own world, but I am doing it one region at a time.
Check out the tutorial section - there are a lot for GIMP because we have quite a few GIMP users here. You also might want to try searching the forums for GIMP tutorials, because some get buried when people haven't posted on them in awhile. (Also, try looking up "award winner tutorial" for some of the best.)
Welcome to the guild!