One can only go with the current best understanding of the data compiled so far, and that understanding says that gravity waves have nothing directly to do wth photons. In string theory, there is some deeper connection between everything, but so far no such connection is useful or predictive. At the level that most problems are treated, it is important to distinguish a graviton from a photon, and there is not a need to "couple" them. I have no idea what the science of gravitons and photons will look like a thousand years from now, but I do know that precise language is required to use science effectively.