I think it looks fine except for the bright white bevel (might go for something grayer). For some dirt I can use a scatter brush in PS but I don't know if you have that in Gimp. So I'd think about making some general shapes for dirt patches, put some noise on that, blur the edges of the shapes to reduce sharpness, and then give it a color overlay...maybe some clouds for variation as well. But, again, I think it looks pretty good.