Not much is known about this zombie, and there is pretty much no screenshots of it that exist.
As suggested by its name it can manipulate gravity and send survivors flying if you go near it. As this information is unconfirmed, take it with a grain of salt.
A place where the gravity zombie may spawn is the Cylinders, seen in the picture.
One user has reported that killing one drops a gun named The Bouncer, but like above these reports are unconfirmed.