Green Glow Orb
To create an orb in Gimp take a perfectly squared image. Click on Filters -----> Distort ----> Polar Coordinates. Keep Circle Depth at 100% -----> Uncheck "To Polar" box -----> Choose OK. Next step is to chose Layers -----> Transform ----> Rotate 180. Click on Filters ----> Reshow Polar Coordinates and this time CHECK the "To Polar" box. Choose OK.
Here is another and probably much better Gimp amazing circle tutorial
There are also a couple of Gimp scripts that will auto make your orb or amazing circle.