Did you know it ? PHP GD's imagecolorallocate() can only allocate 255 colors in a palette based image created by imagecreate() (no such limitation when you use function imagecreatetruecolors() by the way).
This sounds pretty stupid as well as rather undocumented, but anyway, here is a new version of my GD Gradient Fill PHP class, which now produces correct colors for images larger than 256 pixels (hence containing gradients with more than 256 colors)