Select {Image - Apply image} to apply the image of the light pollution - effectively you're
subtracting that from the initial image. "Source" is the light pollution image, "Layer" is
Background, "Channel" is RGB, "Blending" is Subract, "Opacity" is 100%, "Scale" 1, "Offset" 0.

Again, play with Opacity, Scale and Offset to taste (mainly opacity and offset). You should see a
dramatic reduction in light pollution.