Whether an obvious explanation for the equivalence of c and cg exists within the context of GR is a valid question, though of course what is obvious depends on the observer, so to speak. While it is true that in quantum field theory the photon is massless, and hence propagates at c, this does not follow from first principles, but is an assumption, shored up by much observational evidence. There are also more observational tests which have bearing on this issue. In summary, the speed of light is the same as the speed of gravitational waves: c = cg follows from GR and is also verified experimentally, since the arrival times of electromagnetic and gravitational waves are essentially the same (this conclusion is supported by the lack of observed dispersion in GWs). The same observation also rules out some modified-gravity theories which predict different Shapiro delays for photons and GWs. Like Solar System dynamics, while not testing directly whether c = cg, this increases out confidence that GR is correct, in which case c = cg. The speed of light is also the same as the `speed of gravity': c = cG is also built in to GR (there are, however, theories where this is not the case). In both cases, the sole remaining, but subjective, question is whether there is an `obvious' explanation.