Sorry, still don't understand. How about explain in in physics for dummies terms. Never heard of an anti-photon.
You seemed to have no trouble understanding snudge's notion of an "anti gravity particle"; in fact you chided me for not being so clear. Why is the notion of an "anti electromagnetism particle" harder to grasp?
My point is that we don't need to talk about "antiphotons" because they are, by symmetry, the same thing as photons. So, too with "gravity particles" (i.e. gravitons).
Allow me to give it a shot. "Physicist" is saying that there is no particle to "cancel out" a gravity particle's effects, just as there is no particle to cancel out the effect of a photon. The reason there is no such particle is because the particle IS it's own "anti-particle" (self-conjugate, as "Physicist" characterized it).
A crude analogy is that "0" is it's own additive inverse. There is no number you can add to zero to get zero (except for zero itself!) Similarly, a gravity particle is ALSO it's own "anti-particle" (same for photons), which excludes the possibility of some OTHER particle possessing anti-gravity properties from being the gravity particle's "anti-particle."