Gradient effects with GIMP
Raised effect
Now you understand segments and how to edit them this should be easy. Create a two segment gradient by double clicking on the The active gradient on the main GIMP toolbar and then choose New gradient

Right click on the gradient colour and choose Split Segments at Midpoints. This now gives you a two segment gradient.

In the left segment create a dark to light gradient and in the right segment create a light to dark gradient as shown below.

Now apply the gradient on a blank image. The images below are the gradient full length, from the top left to bottom right corners and the same gradient applied over 50px and 10px lengths. I've shown the direction and length of the gradient in white.










