@JohnDuffield "In GR there isn't a such thing as gravitational mass in general. The source of gravity in GR is the stress-energy tensor, which is a rank 2 symmetric tensor (i.e. it has 10 independent compnents). You cannot distill that down into a single scalar number in general. The reason this is important and makes answering the rest of your question difficult is that light, in particular, has a stress-energy tensor of a form where there does not exist any reference frame where all of the terms but one vanish. So there is no such thing as the gravitational mass of a plane wave of light."

@JohnDuffield ". inertial mass

The “m” thing in Newtonian mechanic; thought to be a conserved scalar quantity. For low speeds approximately equals to the rest mass, but generally an obsolete concept since relativistic dynamics. Depending on context, is superseded in relativity either with the rest mass or (along momentum vectors) with 4-momenta. One can’t consistently define inertial mass of a photon, for example." http://physics.stackexchange.com/a/143640/25851 now you can attack their character all you want, but it doesn't change the argument