  We have calculated the opacities and secondary production mechanisms of high
energy photons arising in gamma-ray burst internal shocks, using exact
cross-sections for the relevant processes. We find that for reasonable choices
of parameters, photons in the range of 10's to 100's of GeV may be emitted in
the prompt phase. Photons above this range are subject to electron-positron
pair production with fireball photons and would be absent from the spectrum
escaping the gamma-ray burst. We find that, in such cases, the fireball becomes
optically thin again at ultra-high energies ($\gtrsim$ PeV). On the other hand,
for sufficiently large fireball bulk Lorentz factors, the fireball is optically
thin at all energies. Both for $\gamma\gamma$ self-absorbed and optically thin
cases, the escaping high energy photons can interact with infra-red and
microwave background photons to produce delayed secondary photons in the
GeV-TeV range. These may be observable with GLAST, or at low redshifts with
ground-based air Cherenkov telescopes. Detection of the primary prompt spectrum
constrains the bulk Lorentz factor, while detection of delayed secondary
gamma-rays would provide a consistency check for the primary spectrum and the
bulk Lorentz factor as well as constraints on the intergalactic magnetic field
strength.
