Here's what I've been doing lately for roofs...pattern overlay plus gradient overlay (set to overlay mode). The pattern overlay I got at Dunjinni, it's not perfect since the shingles go up one side and down the other but you get the idea, the gradient is pretty simple. I've attached some pics to help you see how I did it in Photoshop so that you can convert it over to Gimp. The first pic is the final, the second is how the gradient looks without the pattern under it and the third is how the gradient is made: black at one end (50% opacity) to medium gray at about middle (but not quite) where it goes solid white (100% opacity) to medium gray at the other end. You may have to tweak the gradient according to light source (for this the light is from the top). Once you have this done, you can then rotate the house to fit where you want it.