I used semi-transparent gradients on different layers. Mostly a white to transparent gradient. I use the oval select to make the right size oval on a layer above the base circle layer, then while the circle is still selected, start to fill it with the chosen gradient until it looks right. Then I usually apply a little gaussian blur around 4 or 5 to the semi-transparent oval layer. There are some links in your
Does anybody here use gimp thread to tutorials that will help you get started with using the gimp. It becomes alot easier when you learn how to use the layers and other tools the gimp has to offer.
P.S. If you download the xcf I have in the link above, load it into the gimp and take a look at how I have the layers set up, and what is on each layer. That may help a bit too..