Seeing that this is entirely done with RPG Citymap Generator and you're still learning GIMP, this is pretty impressive. If you still want to keep this gray-scale (if using shades of gray work), you could use white, black, and then 3 shades of gray - high, mid, and low to differentiate different elements. Streets could be mid gray, buildings could be dark gray, rivers could be light gray, and you could use black for outlines. A few more shades for the forest and possibly shadows, and you'd have a good gray-scale map that's easy to read. Textures can be helpful too, but with such a large city, and only using gray-scale, textures could also overwhelm or confuse the elements. I don't know where the tutorial is now (saved it, forgot where I found it on the forum) but a great city tutorial by Ravi, "A Guide to the Creation and Depiction of Fantasy Cities" has a fantasic section on positive and negative space, using just BW/gray-scale. Overall, though, this map is good. You're doing an excellent job both with the map and GIMP!

- Kes