I don't know about Kepler but wasn't Cavendish the first recorded person to measure gravity? in like the 17th century? but I think that was just that mass generally has gravity, ie distorts spacetime (he didn't call it that of course) not the local gravitational acceleration of the Earth. I think Newton generally measured that. and a bunch of people also at the end of the 17th century refined the actual measurement. (and they also noticed that it was *different* at different elevations, destroying my guys "gravity is an inertial force due to upward acceleration" hypothesis)