Recall the recent gravitational-wave event caused by colliding neutron stars, which also emitted light. That has enabled scientists to measure the propagation velocity of gravitational waves to a very accurate degree, and so close to the speed of light, as Einstein had predicted
https://phys.org/news/2017-11-physicists-rapid-bounding-gravity.html
https://phys.org/news/2017-11-physicists-rapid-bounding-gravity.html
Just two days later (and after the physicists mentioned above wrote their paper), another paper was published in The Astrophysical Journal Letters by the LIGO and Virgo collaborations, whose authors are affiliated with nearly 200 institutions around the world. By using data from the gravitational waves emitted by a binary neutron star merger detected in August, they were able to constrain the difference between the speed of gravity and the speed of light to between -3 x 10-15 and 7 x 10-16 times the speed of light.