Gravity cat was a small-time internet meme, spawned from the "logic" of strapping toast, which always lands butterside down, to the back of a cat, which lands on its feet, causing a gravitational impossibility.
http://toastcatcaughtinantigravityhell.ytmnd.com/
Then it caught on and some still images of cats floating or upside down under variations of the name "Gravity cat" popped up on LOLcats. Then Gravity cat was invented and used in a couple of Dasboschitt's Garry's Mod videos on YouTube.
And when I joined Fakku, Gravity cat was the first name I could think of.
Yeah, I'm a slave to trends.