Lately I've been using my "quick and dirty picture enhancement" trick quite often. Today I got an idea to combine it with the "gradient selection with quickmask" tutorial. So, here it goes:

  1. Duplicate layer.
  2. On bottom left corner of the picture area on GIMP there is so called 'quickmask' button. To use it, make sure you select all: 'Select > All'. Then click the quickmask button.
  3. Use 'Blend tool' with linear 'Gradient' to select the background.
  4. Click the quickmask button again, and the mask turns into selection. Now make sure you get the background selected. Click 'Select > Invert' if necessary to change the area selected.
  5. Run ‘Colors>Auto>Equalize’ for the top layer.
  6. Try to change the top layer opacity to find a pleasing amount.

As a result you should get more light to the dark background. Alternatively you can use round or other form of gradient selections depending on where your main subject is.