[ohenry]

If Gimp can do layer masking, the fastest method is to add an adjustment layer that darkens the entire image, then add a layer mask to that layer, fill the layer mask with black (which hides all adjustments), then using a paint brush with white selected as your foreground color, make sure you have the layer mask selected, then paint in the areas that you want to darken (areas with white in the mask reveal adjustments). The results are that you'll darken the areas that you selectively paint in. You can do similar layer adjustments with another layer, stacking the layers.

I can't give you more specifics since I don't use GIMP.