I'm not a Gimp user, but I can provide some general pointers. I think I'd find an image somewhere looking up through the trees—a Google image search for "forest canopy" turns up several likely candidates in the first page. Then I'd use some kind of threshold or contrast control to turn it into a black-and-white matte and scale it so that it fits nicely over the map. Change the blend mode to darken or multiply, then adjust the opacity until it looks nice. Maybe add a bit of a blur on it to soften the shadows. Then I'd make a copy of that layer, tint it green, and mask it out entirely. Then I'd find another, similar image and do the same thing, but use it on to punch out the mask of the green-tinted version, to give it a little bit of transmitted green light from the leaves, again adjusting the opacity to taste.
In Photoshop I'd use adjustment layers with the canopy image as a mask, but I am not sure if Gimp has adjustment layers.