Yes, tons of it. They have even filmed light moving about at those speeds. It involves some pretty cool tech:
I don’t see why you would think it is unlikely. Both light and gravitational waves lack mass and are propagating through the same spacetime, so I would expect them to travel at the same speed.
Edited to add:
There is a pretty cool experiment that anyone can do using a chocolate bar and a microwave. You can get a rough measure of the speed of light at home: