Check the tutorials section. In the PDF listing in the sticky entries at the top, there's at least one GIMP tutorial on towns. On the first page of the tutorial section (at least, when I wrote this), there's a tut on medieval cities in photoshop. I dunno what version of photoshop it uses, but photoshop tuts are usually adaptable to GIMP.

That's just what I noticed at a glance. I'm reasonably sure there's more if you dig