>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?

I wanted to read the story, here's a link: https://www.eyeofmidas.com/scifi/Turtledove_RoadNotTaken.pdf

Harry Turtledove also wrote a series of alternate history novels (called the world war series) based on a similar idea. In it, the aliens’ civilization progresses very slowly. A few centuries of human progress would take them millennia. They send a probe to earth in the middle ages, mark earth for an easy conquest, and show up during world war 2 where they find an almost evenly matched adversary. It’s pretty contrived, but an easy and amusing read.

And how do our machine guns fare against a single ship traveling at the speed of light colliding with the earth?