Anti-gravity technology is an extremely advanced technology created by Da'at Yichud. The technology was kept in the safe keep in Roswell, New Mexico in which becoming Area 52 later on. During the losing war against the Nazis, Da'at Yichud gave the technology to the US government to turn the tide of the war.
The application of this technology into visible weapon was met with failure in the initial stage with the experimental aircraft equipped with gravity engine crashed in Roswell.
Despite the failure, the US government continue to further testing and perfect the technology into weapon. However, it was too late after the Nazis dropped the Atomic Bomb on New York City forcing them to surrender and the technology fell into Nazis hand.
In 1961, the Nazis still continue to perfect the technology for their war machine agenda. Unfortunately for them, BJ successfully destroys Area 52 with the mini-nuke, which in turn preventing them to perfect the anti-gravity technology in the long run.