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."
I always thought that you either had light or no light (eg darkness). So - it is either/or 0 or 1. Nothing or something.
a solid wall, which blocks light, would to me be an "anti-photon", or a light blocker, or a light insulator.
To me, light is definitely something, therefore not a zero. And therefore, as a '1', 1+1=2 light photons.
You mention 'gravity particles'. Are there such things?