This is really quite clear; I don't understand the confusion on the part of the other Freepers who are puzzling over this statement.
ANSWER:
The subordinate clause "...photons with up to a billion or more times the energy of visible light..." is really just an extra / unneeded explanation of what gamma rays are. So DELETE it! Sentence then reads:
If you could see only gamma-rays, the Moon [as viewed from the Earth] would be brighter than the Sun [as viewed from the Earth]!
NOTE: I've added the words "as viewed from the Earth" because I am assuming that, were both the Sun and the Moon placed at the same distance from the viewer, the Sun would still outshine the Moon - even in the gamma spectrum. This is the difference between the apparent magnitude of a celestial object (i.e., how it appears to a viewer stationed on Earth) and its absolute magnitude (i.e., its brightness at a set distance).
The meaning of this passage is then obvious: In the visible spectrum, the Sun has a greater apparent brightness (i.e., we here on Earth receive more visible photons from it) than the Moon, but in the gamma ray part of the spectrum, the Moon has a greater apparent brightness (i.e., we here on Earth receive more gamma photons from it) than the Sun (maybe only slightly brighter - the "billions or more times" refers to the more-energetic nature of a single gamma photon in comparison with a single photon of visible light).
The article does the reader a great disservice by not immediately explaining how / why a thermonuclear furnace like the Sun does not emit more gamma photos which reach Earth than the Moon.
Regards,
from an aging webpage:
http://today.slac.stanford.edu/feature/gammaraysfromthesun.asp
> Gamma Rays from the Sun: A New Way for Looking at the Solar System — Until now, gamma-rays emitted directly from the sun have been detected only during rare intense solar flares. However, a paper to be published by Igor Moskalenko of Stanford/SLAC, Troy Porter of SCIPP/UCSC, and Seth Digel of SLAC — in Astrophysical Journal Letters in December — finds that collisions between cosmic-ray electrons and solar photons (sunlight) make the inner solar system a relatively-bright, diffuse source of gamma rays with energies 100 million to 1 billion times greater than visible light. Although the intensity is greatest near the sun, the entire sky glows faintly in high-energy gamma rays from this effect, which is known as inverse Compton scattering.