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?
No.
"Physicist" was trying to explain to you that "regular" particles have "mirror image" (in a manner of speaking) counterparts, called sometimes called "anti-particles." The counterpart to the electron is called a positron. It has exactly the same properties of and electron, except its electric charge is the positive instead of negative; thus the charge "cancels" that of an electron.
In the case of some particles that have no electric charge, the mirror-image particle turns out to be the particle itself. This is the case for photons, and, as "Physicist" has been patiently trying to explain, under any prosed formulation of quantum gravity, for a gravity particle. In such cases where the particle is its own conjugate (self-conjugate), they do not cancel each other out. That's the whole point.
"Physicist" is trying to explain that there is NO theoretical basis on which to hypothesize an "Anti-gravity particle" (which would posess the property of "cancelling" gravity), as this role is reserved for a particle's anti-particle, but when that role is played by the particle itself (when a particle is self-conjugate), there can be no cancellation effect, because in that case the anti-particle possesses the SAME, not the opposite, characteristic needed to cancel that of the regular particle.
I regret having attempted to explain Physicist's point; I seem to have caused more confusion than clarity.