  We have modeled, tested, and installed a large, cerium-activated Gd2SiO5
crystal scintillator for use as a detector of gamma rays. We present the
measured detector response to two types of incident photons: nearly
monochromatic photons up to 40 MeV, and photons from a continuous Compton
backscattering spectrum up to 200 MeV. Our GEANT4 simulations, developed to
determine the analyzing power of the Compton polarimeter in Hall A of Jefferson
Lab, reproduce the measured spectra well.
