Quote Originally Posted by Hai-Etlik View Post
The core of that looks like a continuous noise function like Perlin used as a height field for a lighting filter with a high specularity.
That might require some translation.

Perlin noise is what is made by the Clouds filter. The clouds can be used as the bump map layer in the Lighting effect. I'm not terribly familiar with Gimp, so I can't give you specific instructions, but hopefully that gives you a place to start looking for the answers in the documentation.