What would harnessing of gravitational waves look like?

To put this in perspective, consider a fifty-plus year old human whose main source of exercise for the last twenty-plus years has been walking back and forth between work and the parking spot where the person parks his or her car. Let's strap that person to a bicycle connected to a generator. The energy output of that feeble source of energy easily exceeds the feeble 200 watt gravitational waves produced by the Earth's orbit about the Sun.

Gravitation is extremely feeble compared to electromagnetism. The electrostatic repulsion between a pair of electrons is 10⁴⁵ times stronger than is the gravitation attraction between a pair of electrons. Gravitational waves are in turn extremely feeble compared to gravitation itself. I mentioned the Earth's orbit about the Sun. The total mechanical energy of that orbit is about 10³⁴ times greater than the paltry amount of energy lost due to gravitational waves.

Imagine a Kardashev level III civilization, a civilization that has learned to harness the equivalent of the energy output of an entire galaxy. Aside: Humanity isn't even at Kardashev level I. That's another few hundred years in the future. Also note that the Kardashev scale is logarithmic. Karadshev level I is near future science fiction. Kardashev level II is also in the realm of science fiction. Kardashev level III? That's beyond science fiction.

Even that Kardashev level III civilization would not harness gravitational waves. It would instead go out of their way to avoid them. A Kardashev level III civilization might, for example, intentionally feed matter to a supermassive black hole at the heart of a galaxy to make that black hole become an active galactic nucleus. The energy output of an AGN is immense, exceeding that of a normal galaxy. The gravitational waves produced by an AGN are minuscule compared to the electromagnetic output, by a factor of about 10^-80 or so.