Copy the shadow layer in as a top layer in your gimp document. Turn it's mode to "Multiply" just above the layers in the layer window. Then you can put your texture on the layers under the shadow map and it will shine through the shadows. You can of course change the opacity on the shadow map too if you don't want it as strong.
You might get nicer results with other modes, but i usualy find the multiply work best, it is a matter of trial and error. Have fun with it!