Great, light is electro-magnetic radiation, Maxwell’s equations, blah blah, we get c. Why does gravitational radiation travel at c? I’m all at sea. Obviously, just saying that c appears in Einstein’s equation (with source) isn’t enough. So really the question is why c appears in the metric of space-time as opposed to any other constant with dimension of velocity.
Note that the space-time metric and c_g appears in Maxwell via the * operation regardless of medium, EM gives us mu and epsilon which are medium dependent. This discrepancy leads to dispersion of light and Cherenkov radiation.
So why do mu and epsilon for vacuum yield c_g, experimentally correct to 10^-9? Why is there no Cherenkov radiation in vacuum, immediately after light is emitted from matter? That is the mystery I would like you to resolve.