The script-fu actually leaves the greyscale heightfield as a layer under the coloured one. You could just use that

But if you've already got FT, it can do this sort of stuff right out of the box, no?

I don't have FT, but random HF generation is pretty basic, even Wilbur does it (and much better than GIMP), and my understanding is that Wilbur was the start code fro a lot of FT.

-Rob A>