how would i go about creating a navigation menu like the one at www.gateway.com ?

All it really is is a dropdown list menu, only they've added images to it. Look at their code

I recommend you install the firebug add-on on Firefox and analyze the CSS code for the effect.

You can take the reference of below mentioned URL for making navigation menu. It provides you in-depth information:

http://www.thesitewizard.com/wizards...tionmenu.shtml

Like everyone says, with firebug you can work out how it's done.
Or you could adapt something like this:
http://www.cssplay.co.uk/menus/image_dropdown.html