Using Masks and Layers—Painting, Filling, and Color Tools - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 185

Now activate the new layer and change the Mode setting in the Layers
dialog from Normal to Multiply . Now a multiplication effect is effective on
the two superimposing layers, the image becomes darker and more details
emerge.
Should the image still be too dull or too bright, keep duplicating it again
and again. The duplicates will be in Multiply mode automatically. You can also
duplicate the layer by clicking the icon at the bottom of the Layers dialog.
If your image is too dark after the last duplication, simply reduce the
opacity of the overlying copy and set the brightness and contrast as you
please. When you are satisfied with the result, right-click and select Merge
Visible Layers from the context menu.
Figure 3.12
The edited image with the settings in the Layers dialog