A nuclear photovoltaic battery uses scintillator to convert radiation into visible light, which is then collected by a photovoltaic (PV) cell to generate electricity. If the source of radiation is external gamma-rays, the battery might also be referred as gammavoltaic battery. In this study, a polycrystalline CdTe solar cell was optically coupled with a 2.0 cm × 2.0 cm × 1.0 cm Gadolinium Aluminum Gallium Garnet (GAGG) high light yield scintillator, and the resulting device was tested using high gamma radiation fields from a Cs-137 (1.5 kRad/h) and a Co-60 (10 kRad/h) irradiator, respectively. Measurements with Cs-137 provided a maximum power output (Pmax) of ~288 nW, with a short-circuit current density (Jsc) of ~1.22 μA/cm2 and an open-circuit voltage (Voc) of ~0.34 V. In contrast, Co-60 irradiator gave a Pmax of 1.5 μW, with a Jsc of ~4.73 μA/cm2 and a Voc of ~0.38 V. The CdTe was also coupled with a Lutetium-Yttrium Oxyorthosilicate (LYSO) crystal and tested with the Cs-137 source. The experiment presents a scalable option to reach to high power outputs by harvesting gamma radiation fields in many cases where high radiation field demands heavy shielding and is also considered as unwanted waste.