bfollinprm, I disagree, rest mass is a perfectly well-defined quantity even in general relativity. Granted, you cannot define the rest mass of light by going to an inertial frame, but you can define rest mass to be the invariant length of a particle's four-momentum (mod the sign of your metric), and it will be perfectly well defined and Lorentz-invariant for any particle in both GR and QFT.
To the original poster, as bfollinprm said, there's no rest mass - your expression for momentum is simply incorrect. And in regards to coupling to gravity, it isn't just mass that couples to gravity, energy and pressure cause gravitational interactions. Since mass is a form of energy (E=mc^2), it dominates gravity for nonrelativistic objects, but for objects moving close to/at the speed of light, or objects with enormous pressure, there are additional gravitational terms. This is how light can cause a gravitational pull.