  We study high-energy gamma-ray afterglow emission from gamma-ray bursts
(GRBs) in the prior emission model, which is proposed to explain the plateau
phase of the X-ray afterglow. This model predicts the high-energy gamma-ray
emission when the prompt GRB photons from the main flow are up-scattered by
relativistic electrons accelerated at the external shock due to the prior flow.
The expected spectrum has the peak of 10-100 GeV at around the end time of the
plateau phase for typical GRBs, and high-energy gamma rays from nearby and/or
energetic GRBs can be detected by current and future Cherenkov telescopes such
as MAGIC, VERITAS, CTA, and possibly Fermi. Multi-wavelength observations by
ground-based optical telescopes as well as Fermi and/or Swift sattelites are
important to constrain the model. Such external inverse-Compton emission may
even lead to GeV-TeV gamma-ray signals with the delay time of 10-100 s, only if
the plateau phase is short-lived.
