  Recently, Fermi-LAT detected GeV emission during the X-ray flaring activity
in GRB 100728A. We study various scenarios for its origin. The hard spectrum of
the GeV emission favors the external inverse-Compton origin in which X-ray
flare photons are up-scattered by relativistic electrons in the external
forward shock. This external IC scenario, with anisotropic scattering effect
taken into account, can reproduce the temporal and spectral properties of the
GeV emission in GRB 100728A.
