If you're doing this in gimp, then the easiest way to get a good parchment look is to find a good parchment texture and put it in the layer below your buildings, and then set your building layer to multiply. Or you could put it on a layer above your buildings on a white background and set the blend mode to overlay. Either should work (I think).

Check out the mapping elements section, there are lots of good parchment pre-made textures there and some good tuts as to how to make your own.