  We calculate the spectrum of blast wave models of gamma-ray burst sources,
for various assumptions about the magnetic field density and the relativistic
particle acceleration efficiency. For a range of physically plausible models we
find that the radiation efficiency is high, and leads to nonthermal spectra
with breaks at various energies comparable to those observed in the gamma-ray
range. Radiation is also predicted at other wavebands, in particular at X-ray,
optical/UV and GeV/TeV energies. We discuss the spectra as a function of
duration for three basic types of models, and for cosmological, halo and
galactic disk distances. We also evaluate the gamma-ray fluences and the
spectral characteristics for a range of external densities. Impulsive burst
models at cosmological distances can satisfy the conventional X-ray paucity
constraint $S_x/S_\gamma \siml$ few percent over a wide range of durations, but
galactic models can do so only for bursts shorter than a few seconds, unless
additional assumptions are made. The emissivity is generally larger for bursts
in a denser external environment, with the efficiency increasing up to the
point where all the energy input is radiated away.
