I'm not sure what GIMP is, or what it is capable of, but I can say that those textures do look kind of plasticy. It looks like you have filled an area, applied some noise and a few filters, and got a finished product. Obviously this is how you're going to start, but I'd recommend doing more by hand. Use the darken/lighten brush, if there is one in GIMP. Usually layering multiple images and experimenting with different blend modes will give a better effect. Also experiment with the image contrast and brightness. Don't refrain from using filters completely, some effects will be a lot easier and quicker to do using filters, but don't rely soley on them. Noise is good for adding depth to the texture, just dont be too harsh, and try to blend several layers of noise together.
In the end, as with anything practice makes perfect. I'm no expert texture artist, but I got to where I am by simply experimenting with the tools available. Tutorials on the internet occasionally help, though most of the ones I found are far from perfect, in fact a lot of them were pretty damned awful
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Isn't it? Wasn't it? Marvellous!
