I must warn that though I'm argumenting with black holes I'm not asking how does gravity escape the black hole!. I want to know if the absolute speed of gravity waves were proven bu an experiment.
We know that anything with $v\le c$ cannot escape the event horizon of a black hole. But the gravity itself seems quite unaffected by itself. There can be the explanation that the gravity does not interact with itself - but no matter what, in this case it directly violates the rule that no information can leave the black hole.
This brings me to he question itself - I'm having serious doubts that the speed limit was proven for the gravity.
I've found nice explanation of the problem with the black hole though:
As I understand it, to the outside observer the black hole takes an infinite time to form, although in its own frame, the black hole forms right away. In any case, the gravity felt by the outside observer is that of the object becoming a black hole.
