Gamma-ray-induced photoemissions from GaN are investigated for as-grown and gamma-ray-irradiated single crystal wafers at room temperature. The GaN crystals are irradiated with gamma rays of 1.17 and 1.33MeV from a cobalt-60 source. The gamma-ray-induced photoemission is analyzed by a spectroscope using the optical fiber cable placed in front of GaN. In the gamma-ray-induced photoemission from as-grown GaN wafers, the yellow luminescence (YL) is observed with a peak at around 600nm in the spectrum around 440nm-900nm. The main YL peak is attributed to the Ga vacancy. The emission intensity increases almost in proportion to the wafer thickness when the thickness of the single crystal wafer increases from 0.25mm to 0.75mm due to the strong penetration of gamma rays in the crystal. In He-Cd laser excitation, the emission intensity does not depend on the thickness of the sample because the penetration of He-Cd laser light is near the surface. On the other hand, a gamma-ray-induced photoemission from GaN irradiated with a total gamma-ray dose of 990 kGy is observed with a peak at around 700nm in the spectrum around 450nm to 850nm. The energy of the emission peak is close to the transition energy (1.83eV) from the +1 state to the +2 state of the Ga interstitial. The gamma-ray-induced photoemission from GaN at room temperature suggests the possibility of GaN single crystal wafers to be used as a gamma-ray detector.