Research of high-efficiency ambient temperature photon sensors is ongoing due to the demand on measuring x-ray and gamma-ray radiation with high energy resolution. The inorganic wide-bandgap perovskites such as CsPbBr₃ have high mobility-lifetime product (10⁻² cm² V^-1), low defect densities, and long-term stability for photon and charge particles detection. These detector materials are promising for large crystal designs. The study of large-size perovskite detectors is necessary, including development of computational models. The performance of a gamma detector based on the CsPbBr3 crystal was studied using GEANT4 and ROOT toolkits. The Monte Carlo GEANT4 code utilizes geometry and materials data to model particle interactions with matter, event and track management, and visualization of results. The ROOT was used to process and analyze the gamma-ray energy distributions computed by GEANT4. The 8 cm³ CsPbBr₃ crystal characteristics for the incident 662-keV gamma rays were the following: 1.1% energy resolution and 29.2% photo peak efficiency. The energy resolution of the perovskite detector is comparable to that of a CZT detector of a similar geometry; however, the larger size perovskite detectors can be fabricated.