Take a look at Stafan Morrell's Hard Surface Texturing tutorial.

It's a little involved, and I'm pretty sure you can't get an ambient occlusion map out of Sketchup, but the techniques to get grunge and scratches are superb, and they should work just fine in Gimp.

There was also a tutorial on painting hull plating posted here a while back, I think in another Sketchup thread, but I don't feel like digging it up at the moment. Seems like RobA posted it, but my memory fails me.