G=E/M. The M stands for Mass, btw. Since E=MC^2, we know that the speed of Gravity is C squared (E/M=C^2; G=E/M; E/M=E/M; therefore C^2=E/M and E/M=G so G=E/M).
What you've got to understand is that Gravity exists because of motion/speed. Once something moves fast-enough, it begins to emit Gravitons. Even something Massless, such as Light, emits Gravitons so long as it is moving fast-enough. In this way, Light is affected by Gravity. Watch the Light as it bends around the edge of your thumb if you hold it to up to a light-source, for instance. Greater masses bend Light even further, such as planets or Black Holes.
But only up to a certain point. Slow light down to a fast run, for instance, and Gravity no longer bends that light. The recent experiments in which Light has been slowed down to under 30 miles per hour confirm that fact, too.