GIMP or not to GIMP...
GIMP is, for what it does, highly powerful, and very awkward to learn.
There are stamps for GIMP that were designed for doing RPG mapping at an "old school D&D style map" level, both color and grayscale...
And a tutorial for you...
Vector is better...
For macro-level mapping, you're better off using a vector graphics editor of some kind. The best one I've found for free is Inkscape, but OpenOffice's draw module should work, too. Another option, also free, is sketchup.
Vector is a better option because you can scale it far more easily, and with no nasty rasterizations.
3 functional practices make it very practical to map in any decent editor:
- distribution of duplicates
Very few programs do distribution automatically other than by grid, and while there are a few plug ins to do it... it's best to use a program designed for the job.
Layers are essential for organizing data. If you have all your polity data in one layer, roads in another, and terrain in a third, you've a good start..., but you can easily add a 4th for GM notes...
Groups allow you to create reusable elements easily.
The best for city mapping I have found isn't free... it'll run ya $60... CC3 and City Designer 3. Has a wide variety of mapping symbols, including randomized strings of buildings, and is a full fledged 2D CAD program. It's not as bad to learn as CC2 was; it's still non-trivial, though. Lots of map symbol sets, runs well, does run in WINE fairly well for non-Windows Users.