No one's talking about perpetual motion here. Understanding the laws of thermodynamics allows us to build heat engines, which is how many of us got to work this morning. It also allows us to build nuclear reactors (or at least the steam-cycle part of it), which is how many of us get the power to write messages here.
What does our "understanding" of gravity allow us to do technologically? Nothing, because we have zero idea of how to manipulate it. The only thing we can do is understand how it works in the natural world so we can, for instance, navigate space probes accurately and get our GPS satellites to work. That's great and all, but it falls far short of the level of understanding that we have with thermodynamics and electromagnetism.
Yes, that's an application of Maxwell's laws of electromagnetism. That's not manipulation of gravity, it's using the MUCH stronger EM force to overcome gravity.
I can overcome gravity all by myself just using my muscles to lift things. That doesn't mean that I've manipulated gravity in any way.
I'm really shocked that I seem to be the only one who groks the difference between observing a natural physical force and actually understanding it well enough to manipulate it and generate it at will. We cannot generate gravity. We can generate EM fields, and we can also generate nuclear energy by splitting or fusing atoms (which means we're manipulating the nuclear strong force, to an extent).
There's your misunderstanding. Gravity is not a force. The force is merely the effect you observe on your specific instrument. The cause and substance of gravity is not explained solely by its effect (force), and the force is not what generates gravity, et al. And, in fact, mass is not the only thing that generates gravity. It's flux of energy density. So it boggles my mind that no one considers EM energy as a subset of the energy that can produce "stress on space".
Conclusively, you've made some assumptions you don't realize resulting in an uncontrolled thought experiment. When terms are defined incorrectly, questions using those terms become wrong. If the questions are wrong the answers also always come out wrong.
