...but with the way this image has been retouched, the photographer used color adjustment? I am only guessing. >.<

Do you have the original? Someone could try to replicate it for you and then tell you how it was done.

There is a tool called curves, you can find it by going colour>Curves, you are able to adjust the colours of your image by dragging the curve about. You can adjust the value of a pixel or the individual channel colours and the alpha channel.

A quick search of the net for "Gimp Curve Packs" will help you find predefined curves other users have already created for use with images to get certain effects.
Simply download them and place them into your curves folder usually located user/gimp 2.8/curves or similar depending on system.

Here is a LINK to some curve packs.

Also have a quick read through HERE

I would have said curves too. If the colour has just been made richer looking I would have used curves. You can also use this tool to lighten things but looks like her hair is a rich colour.

I would change hair color and skin tone with whole layers that are adjusted and then an airbrush on a layer mask to fill it in to the image. The results are very natural looking variations. You can even colorize bw shots this way. In fact, I made a tutorial on it, a while back, you can find here. viewtopic.php?t=6215&p=77203#p77203


As an aside, my roommate has hair that color this week.