  Gamma-ray bursts are generally considered to be the result of internal shocks
generated in an inhomogeneous relativistic outflow that arises from a fireball.
In such shocks, the Fermi acceleration of protons is naturally expected to be
at least as efficient as that of electrons. We investigate the consequences of
proton acceleration in the residual thermal photon field of a fireball,
especially those on the emission spectra of photons. In contrast to most other
studies, we do not invoke a direct electron acceleration in shock waves. We
show that the residual radiation field of the fireball can ignite the photopion
production and subsequent cascade, and that the photons emitted in this process
further enhance the photon-initiated cascade. We find that Fermi accelerated
protons with $\gtrsim 10^{13}$ eV efficiently bring their energy into pion
production and subsequent photon emission. The particles cascading from the
pions emit photons over a wide energy range. The photons emitted from
electron--positron pairs distribute continuously from the GeV range down to the
X-ray range, while muons can emit gamma-rays by synchrotron radiation with a
break at around 1--10 MeV. We also discuss several radiation processes which
may possibly produce a break feature in the MeV range, as observed in addition
to muon synchrotron radiation.
