  It has recently been proposed that ultrahigh energy ($\gtrsim 10^{19}$ eV)
cosmic rays (UHECRs) are accelerated by the blast waves associated with GRBs.
We calculate the observed synchrotron radiation spectrum from protons and
energetic leptons formed in the cascades initiated by photopion production,
taking into account $\gamma\gamma$ attenuation at the source. Normalizing to
the emission characteristics of GRB~970508, we predict $\sim 10$ MeV - 100 GeV
fluxes at a level which may have been observed with EGRET from bright GRBs, and
could be detected with the proposed GLAST experiment or with ground-based air
\v Cerenkov telescopes having thresholds $\lesssim $ several hundred GeV. The
temporal decay of the UHECR-induced high-energy $\gamma$-ray afterglows is
significantly slower than that of the lower-energy burst and associated
synchrotron self-Compton (SSC) radiation, which provides a direct way to test
the hadronic origin of a high-energy GRB afterglow. Besides testing the UHECR
origin hypothesis, the short wavelength emission and afterglows can be used to
probe the level of the diffuse intergalactic infrared radiation field or
constrain redshifts of GRB sources.
