Well you can say that about anything can't you?

wE dOnT kNoW wHaT rEd iS!

We can name thousands of variations of the color red, we can measure the wavelength of the light that produces the color red, we can build machines and devices that manipulate matter at the quantum level to produce the color red, and yet... it's still impossible to actually say what red is, and no one knows if what they perceive as red is the same as what others perceive.

So ya we don't "know" the ultimate true nature of gravity, and yet...

...even babies and simple animals learn and understand that gravity exists, and we can measure gravity to astonishing levels of accuracy, and we understand gravity enough that we can launch a rocket up into space that then launches a probe that can travel millions of kilometers through space over several years and land on the surface of a comet that's travelling tens of thousands of kilometers an hour, but ya... "we don't know what gravity is"

If you can spend your entire life studying the mountains of data and literature that we have amassed about a topic, then is it honest to say we know nothing about that topic?