Gravitas {pronounced "Grah-Wih-Tahs" if you're speaking the language, and with a "V" in English) is Latin for "gravity." (Just in case you're interested, the "-itas" ending almost always becomes an "-ity" - libertas=liberty, dignitas=dignity, etc.) Given that the Ancients didn't have a completely cohesive understanding of the laws of physics (though they still knew quite a lot), this means also many of the things we today can mean when we say "gravity" - seriousness, solemnity, etc - or just sheer weight.

When used to describe George W. Bush, the meaning is then probably clear - he was a lightweight. Not a real politician, maybe, not someone who cares about the responsibility of governing - Al Gore, however, cultivated an air of seriousness and by extension, dedication to the fairly serious job of politics.