you could try creating a mask (black and white image) concentrating on the pathways than the buildings. here's a quick photoshop test - GIMP will probably have similar things
1: fill background
2: draw out streets
I used a brush to draw the biggest, then keep reducing the brush size for alleys ways etc. also used the circular marquee tool and then stroke white for the curved areas. with Photoshop CS2 and higher you can press shift when using a brush to create striaght lines.
My example is pretty slap-dash resulting in acute angled walls , whitch while do occur in real life in buildings, might be too extreme. more care at this point for more square structures will make a better looking city similar to the example you have.
You can also have a brush of a street, turn up the spacing for regular distanced streets - you'll need to adjust the angle as you go.
3: using 2, I turned it into a mask - putting it in the channels palette and so I could only select the black areas. created a new layer and filled grey.
on that new layer used the layer FX bevel and drop shadow.
4:played with filter High pass and Adjust levels.
it looks like that example uses photocopy or similar. I didnt spend long - about 30 minutes (working out a method, and typing this response inc) Obviously with more care you could get some L, H C shaped buildings.
If you do use Photoshop, you can also create the foot print of buildings even faster by openeing up the square brushes, and brush options. turn up the spacing , the angle a bit and the size jitter. though with this you can only have rows and collumns of buildings. If you have Painter, I think you can probably do the curves streets with this method. I havent used it much but I know it does have a brush to orientate to the angle of stroke. Coral painter usually has free tryB4 you buys to downloads.
You can also use Photoshop another way - rather than the square brush, make one detail, high res version of a building ( or half a dozen types) then turn it into a brush with size jitters and you can rotate it etc as mentioned above. your create a hi res version and so you can scale it as a brush without it pixelating and draw with it with spacing turned up. If you can shift click with this brush for a straight lines then you can create blocks pretty quickly.
hope it helps. I didnt spend too much time on this, so Im sure there may be better ways. for example I'm already thinking - get the Roleplaying City Map Generator to help create the mask and go from there.
or someone else might have a more simple approach still.