This is a simple gradient plugin for GIMP, written in python. It applies a linear gradient to a rectangular selection, in one of four directions, using the colours of the pixels at the starting edge, and fading to the background colour, over the width, or height of the selection. To try this out, all you need to do is download the script, and store it in the gimp plugins folder, in your home directory. Remember to set the executable flag on the file. An example of use may be found on my site : www.andyshelley.co.uk/gimpgradient/index.php
Edit : I have now updated the script, so that the gradient effect appears on a new layer.
Edit: The script is now updated to give extra fading options.