This time I thought I'd do a full tutorial description , as I have tweaked my editing a little bit.
Follow the lime green numbers.
1. In red, you can see an arrow pointing to the gradient tool. Make sure this is selected. The next few red arrows point to things you must change. The mode must be set to "Overlay", the Gradient must be set to "FG to BG (RGB)" (and the little box beside the gradient picture must be check marked) and the shape must be set to "Radial".
2. Click the middle of the photo, hold down your mouse button, drag the mouse to any outer corner of your image, and then release. Then your gradient will show up. It should be lighter in the middle and darker on the outer edges.
3. In the "channels, layers undo" box, select the thumbnail of your image, right click, and scroll down to "duplicate layer". A second thumbnail should appear above the first. Select this thumbnail, and then click the tiny downwards arrow across from the word "Mode" (which is kinda at the top of the channels, undo layers box). Scroll down and click the word overlay, and then set the opacity of this layer to 50%.
4. After the changes in step 3 are made, your image should have well defined highlights and shadows.
5. Go to the top main bar of GIMP, and click the word "Colours" then scroll down to the word "Curves" and click that. Make your graphs look like mine by pulling at the graph curves. Switch between graphs by clicking the arrow beside the word "Channel"
6. Click "OK" at the bottom of the curves box, and you should arrive with the finished product.