>So when the alien invaders arrive with the FTL ships and release their conquering armies...our automatic machine guns and missiles and tanks and bazookas and such totally outmatch the aliens troops who have flintlock rifles and black powder.

That doesn't even make sense. Why would having simple gravity manipulation stop progress on the development of other technologies, particularly weapons?

Especially if, as it seems, those weapons aren't mitigated by having gravity manipulation? Literally no one in the universe realized the potential of having gravity manipulation and modern technology?