As it has been already pointed out a couple of times Gmail uses a whole bunch of iframes for its interface. You can use Firebug to view the source code of Gmail, however Gmail is known to crash your browser and slow it to a crawl when inspecting the page with Firebug.
Although not only does Gmail use iframes, but it uses quite a lot of obfuscated (well, heavily compressed) Javascript code which modifies the DOM after the page has loaded, hence why you would not be able to see the proper contents of Gmail when using your browsers standard view source option.
I prefer using Dragonfly in Opera as it tends to be a bit more stable navigating the contents of Gmail, good luck.