Actually, “Multiply” is not the only mode that is useful for gain control. In fact, “Multiply” mode can only darken parts of an image, never lighten them, so it is only useful where some parts of an image are overexposed. Using “Divide” mode has the opposite effect: it can brighten areas of an image but not darken them. Here is a trick that is often useful for bringing out the maximum amount of detail across all areas of an image:
- Duplicate the layer (producing a new layer above it).
- Desaturate the new layer.
- Apply a Gaussian blur to the result, with a large radius (100 or more).
- Set Mode in the Layers dialog to Divide.
- Control the amount of correction by adjusting opacity in the Layers dialog, or by using Brightness/Contrast, Levels, or Curves tools on the new layer.
- When you are happy with the result, you can use Merge Down to combine the control layer and the original layer into a single layer.
From GIMP manual