you should just upgrade to photoshop. i used to run gimp before, and my images didn't look as good either. it's after i switched to ps, i started to get really good results.
now from a raw point of view, i think you composed this very well. the lighting is nice,
giving the photo a good depth, and the contrast is nicely balanced with the exposure,
with the highlights very much intact and not blown out, except for that area on the corner of the rear bumper, but then again there's not much that could be done there.