  We calculate the flux of "GZK-photons", namely the flux of Ultra High Energy
Cosmic Rays (UHECR) consisting of photons produced by extragalactic nucleons
through the resonant photoproduction of pions, the so called GZK effect. We We
calculate the flux of "GZK-photons", namely the flux of Ultra High Energy
Cosmic Rays (UHECR) consisting of photons produced by extragalactic nucleons
through the resonant photoproduction of pions, the so called GZK effect. We
show that, for primary nucleons, the GZK photon fraction of the total UHECR
flux is between $10^{-4}$ and $10^{-2}$ above $10^{19}$ eV and up to the order
of 0.1 above $10^{20}$ eV. The GZK photon flux depends on the assumed UHECR
spectrum, slope of the nucleon flux at the source, distribution of sources and
intervening backgrounds. Detection of this photon flux would open the way for
UHECR gamma-ray astronomy. Detection of a larger photon flux would imply the
emission of photons at the source or new physics. We compare the photon
fractions expected for GZK photons and the minimal predicted by Top-Down
models. We find that the photon fraction above $10^{19}$ eV is a crucial test
for Top-Down models.

Astrophysics