Step 1: A Gradient to get a subtle light point.
Open the photo you want to give some extra light in Gimp.
Add a new (transparent) layer.
Set the foreground color to white and the background color to mid gray (808080).
Activate the blend tool.
Set the the following blend tool option.
Gradient: FG to BG
Now draw the gradient so you get something like this. To get the nicest result make sure the center of the gradient is close to an eye.
Step 2: The extra light effect
Change the blend mode of the gradient layer to Value. And you’ll get this strange effect.
Duplicate the background layer and place it on top of the gradient layer.
Change the blend mode of the duplicated layer to Addition and you’ll get this.
Duplicate this layer again and change the blend mode to Multiply.
Step 3: Only if the result isn’t good enough
Sometimes the contrast becomes to big. The whites are to bright and the blacks to dark.
Duplicate the top layer and set the blend mode to normal.
And lower the opacity to 10-20.