You can trace the foundation of the claim that the US gov. has anti-gravity craft in the modern age here. It is interesting to note that at the time this was made the US gov. still denied the existence of area 51. Also of note, Lazar was widely ridiculed for his claim that an as-yet undisclosed "element 115" drove the propulsion of these vehicles. That element has in deed been confirmed, reproduced and added to the periodic table.
It's also interesting that, essentially, the warp reactor Lazar describes is pretty much the same as that used by the star ship Enterprise except you replace the dilithium crystal with element 115.
Gravity A and B.
Okay.... but element 115 is Moscovium.... According to wikipedia has a half life of .8 seconds in it's most stable isotope. Other isotopes are even less (.3s, .2s). How do you produce a propulsion system using a fuel with a decay that fast?
I've heard him refer to "a stable sample of element 115". So I would guess they supposedly have a way of rendering it inert.