Gradients are not possible with rectangles. An alternative would be to upload a gradient image and similarly change the layer.
Or if you want to go the HTML route, there are a lot of examples online that you can have a look at and give a try. Here is one I found that you can review -
coolhomepages.com/How-to-make-easy-gradient-shadow-CSS-DIV-boxes/blog-48.html.
And here is a quick and dirty example that I threw together for both of the above -
go.kokopop.com/gradienttest.html