Of course scientist know what is gravity, they know more than that. What DeGrass and a lot of scientist try to say, we can't know everything down to the essence. There will always be something left to be discovered. This is what make the science beautiful, knowing that there is always something else left to be discovered.
Unfortunately, this gets misinterpreted by a lot of people, victim of poor education on some countries or states.

99.99% of what we enjoy today, is because of those "ignorant" scientists that everyday push the frontiers on all fields of science. If it was not for them, we would not have electricity, computers and even steemit to discuss.