It is a given that the speed of light in a vaccuum is the fastest speed there is: light travelling through a medium like air or water is slower due to a time lag of photons being absorbed and re-emitted by atoms.
Is there an absorbsion/re-emission time lag in effect when gravity passes through matter?
Suppose an invisible Black Hole (as opposed to one with a very visible accretion disc ;)) were emitting gravity waves. On the way to Earth, these waves pass through a nebula or dust cloud.
Would a theoretical gravity wave detector sense some kind of lag?
In principle, there could be and probably is an “index of refraction” for gravity. In practice, though, gravity is so weakly interacting that it can always be neglected.