How to add gradient to sheet?

Hi All,
I am trying to implement the design show here: http://img541.imageshack.us/img541/1426/image1xf.jpg
The website’s current status is here: http://tinyurl.com/37zmmwe

The only thing I am unhappy about (ignore the Chinese language) is the gradient of the sheet which I can’t work out how to do.
Does anybody have any advice?

Many thanks!