There were indirect proofs that the speed of gravity is close to the speed of light since at least 1980-es, based on the 1974 discovery of a pulsar in the binar

There were indirect proofs that the speed of gravity is close to the speed of light since at least 1980-es, based on the 1974 discovery of a pulsar in the binary system (1993 Nobel Prize in Physics for Hulse and Taylor) and the measurements and analysis of that data: https://www.forbes.com/sites/startswithabang/2016/04/28/why-does-gravity-move-at-the-speed-of-light https://www.forbes.com/sites/startswithabang/2016/04/28/why-... "we have constrained the speed of gravity to be equal to the speed of light with a measurement error of only 0.2%!" But this one now is the first direct and so precise measurement that there is no more wiggle room for those who needed it for their hypotheses. Written in 2016 in the article I've linked: "The best results, at the present time, tell us that the speed of gravity is between 2.993 × 10^8 and 3.003 × 10^8 meters per second, which is an amazing confirmation of General Relativity and a terrible difficulty for alternative theories of gravity that don’t reduce to General Relativity!" Now in 2017 also confirmed by the direct observation. The Einstein's General Relativity (1915) is more than 100 years old. Those who designed all the detectors for the gravitational waves were sure decades ago. There were already enough consensus even then that allowed that long work, after all, it's all based on the 1915 theory confirmed in many different ways since. But it's so nice that now we have the real direct measurements of the gravitational waves and the EM signals together. It's really something comparable to the first use of the telescope 400 years ago.