Using Masks and Layers—Painting, Filling, and Color Tools - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 186

3.4.3 Editing Underexposed Images
The process for fixing overexposed images works in similar manner with
underexposed images. To follow along, select an underexposed image or the
image underexposed.png in the SampleImages folder on the DVD. The process
I will show you also helps getting the optimum out of strongly underexposed
images.
After opening your image, duplicate the background. This time select the
Screen mode. Duplicate the copy until your image seems overexposed. Now
use the Opacity slider to adjust the brightness of your image: 100% opacity is
the highest level of brightness, 50% is half as bright. Adjust to your liking and
then merge the layer to create one background layer.
Figure 3.13
The image after editing with the settings in the Layers dialog