I hate to break this news but the Gmail Motion beta is actually a prank from Google. I never learnt it was all untrue till i tried clicking the link.
Imagine using Gmail via body language, by virtue of the camera built-in to your computer (if there's any), you could mimick opening an envelope to signal opening an unread mail; pointing your thumb backwards to reply; pointing your two thumbs backward to reply all; and so on...
It was actually another cool advancement in technology, a camera interpreting your body language through motion-detect algorithms. It's really a must try!!!
Much to my eagerness and being a fanboy to the Gmail engineers, i found the video so convincing at first during the first 30 seconds of its playback. But when it rolled down to the guy demoing the body language commands like mimicking to lick a stamp and putting it down - to send a message, it went all from seriousness to laughter (at least in the back of my head). Nevertheless, my urge didn't stop to click the download link in trying the Google Motion. Only to find out that it was just a joke.
It's April the 1st - my first April Fool's prank of the day and i got sold!!! In retrospect, this could happen anytime soon. Google has its way of giving a lil bit what's in store for the future of their giz.