You're making the fundamental mistake of confusing cause with effect, observation and measurement with fundamental understanding.
What are the mechanisms that give rise to gravitational waves? What is gravity? You claim to have the answers?
To date, we only know gravity and gravitational waves exist and its observable relationship with space-time. We don't know what it is and how it works - let alone how gravity waves propagate.
Einstein did not "tell us how gravity waves propagate" at all. His theory of GR predicted that a small side effect of the phenomenon of gravity is that it would produce a wave, like a shock wave that rippled through space. He had no idea what the precise mechanism behind all this was because he was dealing only in the phenomenological aspects, rather than the empirical. The key is understanding what the wave and what gravity
IS. We have a few theories about what space-time is but again, we know scant little about what is behind these phenomena.
You claim that we will be able to make use of gravity someday and that all we need do is "observe it some more". Well that would be great, but is a bit fanciful to think we are anywhere near a proper understanding required to bend gravity to our will.
Have a nice Friday indeed!