To get sand on top of things, just use a scatter brush with soft edges and paint sand into crevices and on top of rocks. I use photoshop, so I'm not sure of the exact process in gimp, but I would either paint white scatterings on a new layer and then fill it with the sand texture/pattern, or use the scatter brush to paint a mask.

Mess it up a bit, just like Kennyt said.

cheers,
Meshon