As you practice drawing basic forms you train your eye to see areas of light and dark. As you draw these areas your forms appear 3D. These basic forms are found in the structure of many things around us.
Here are some Gimp tutorials to try. Click on the link and follow the instructions. Remember Gimp 1.0 is slightly different than Gimp 2.2 which is what we are working with - try to do the tutorials that are for Gimp 2.