A trick I use to give the default texture a more realistic feel is to create 3 layers of the texture. I set each one to multiply then merge all three of them. I then go over with my blur and smudge brushes to soften out some harder edges. Then using the burn and dodge tools, I go over some areas and add highlights and shadows. I find that it normally gives a more realistic edge to it. Hope this helped! c: Also, I use photoshop, so I hope you have the same/similar tools in Gimp!