  The prompt emission of gamma-ray bursts (GRBs) is widely thought to be
radiation from accelerated electrons, but an appreciably larger amount of
energy could be carried by accelerated protons, particularly if GRBs are the
sources of ultra-high-energy cosmic rays (UHECRs). We model the expected photon
spectra for such "proton-dominated" GRBs in the internal shock scenario through
Monte Carlo simulations, accounting for various processes related to
high-energy electrons and protons. Besides proton and muon synchrotron
components, emission from photomeson-induced secondary pair cascades becomes
crucial, generally enhancing the GeV-TeV and/or eV-keV photons and offering a
signature of UHE protons. In some cases, it can overwhelm the primary electron
component and result in GRBs peaking in the 10 MeV - 1 GeV range, which may be
relevant to some bursts discussed in a recent re-analysis of EGRET TASC data.
The dependence of the spectra on key quantities such as the bulk Lorentz
factor, magnetic field and proton-to-electron ratio is nontrivial due to the
nonlinear nature of cascading and the interplay of electron- and proton-induced
components. Observations by {\it Fermi}, ground-based telescopes and other
facilities should test these expectations and provide critical constraints on
the proton acceleration efficiency.
