this is a good concept to explore. http://www.callowayengines.com/ The center of gravity must constantly change in the wheel(s) favor for it to do work using attraction of any kind.
He also mentions that when inventing gravity engines one should start with a setup that wants to start moving.
The setup from David Hamel
looks rather interesting. The Hamel Free Energy Generator created on May 1997 - JLN Labs - Last update September 23th, 1999 JLN Labs - Hamel Free Energy Generator
but it's a lot like that of Billy Meyer.