From the site documentation:
$image = new gd_gradient_fill($width,$height,$direction,$startcolor,$endcolor,$step)
string $direction
The shape or direction of the gradient, which can be any of : vertical, horizontal, ellipse, ellipse2, circle, circle2, rectangle, diamond.
So in the function you set the direction to diamond etc, try out the different ones on your own to see how they display.
EDIT
To make them from scratch, well the easiest way is look at the source code of that script: http://planetozh.com/download/gd-gradient-fill.php from there you should be able to rummage through it and get the proper commands for it. Given it is only 240 lines of code it should be trivial to read through it.