  Relativistic bulk motion with large Lorentz factors has recently been
inferred for gamma-ray bursts regardless of whether they are of galactic or
cosmological origin. This conclusion results from calculations of internal pair
production transparency in bursts that usually assume an infinite power-law
source spectrum for simplicity, an approximation that is quite adequate for
some bursts detected by EGRET. However, for a given bulk Lorentz factor
\teq{\Gamma}, photons above the EGRET range can potentially interact with
sub-MeV photons in such calculations. Hence it is essential to accurately
address the spectral curvature in bursts seen by BATSE. In this paper we
present the major properties induced in photon-photon opacity considerations by
such spectral curvature. The observed spectral breaks around 1 MeV turn out to
be irrelevant to opacity in cosmological bursts, but are crucial to estimates
of source transparency in the 1 GeV -- 1 TeV range for sources located in the
galactic halo. We find that broad absorption troughs can arise at these
energies for suitable bulk motion parameters \teq{\Gamma}. Such troughs are
probably an unambiguous signature of a galactic halo population, and if
observed by experiments such as Whipple, MILAGRO and GLAST, would provide
powerful evidence that such bursts are not at cosmological distances.
