It is clear that gravity propagates at the speed of light in the vacuum. But what happens to gravitational waves as they propagate through material mediums? Presumably they get slowed down but, is it known precisely how much and by which factors, density of the medium or any other properties? And what if the gravitational waves propagate through a region of space which is itself 'warped' by gravity (by say, a neutron star or whatever). Or are they affected for example by heavy electromagnetic fields, in a similar (reciprocal) way as EM radiation is affected (bent) by gravity? In summary, how good is our understanding of the propagation of gravity through mediums other than the perfect vacuum, and through other energy fields? Thanks!