We developed the X-ray, gamma-ray, and relativistic electron detector (XGRE) onboard the Tool for the Analysis of RAdiation from lightNIngs and Sprites (TARANIS) satellite, to investigate high-energy phenomena associated with lightning discharges such as terrestrial gamma-ray flashes and terrestrial electron beams. XGRE consisted of three sensors. Each sensor has one layer of LaBr3 crystals for X-ray/gamma-ray detections and two layers of plastic scintillators for electron and charged-particle discrimination. Since 2018, the flight model of XGRE was developed, and validation and calibration tests, such as a thermal cycle test and a calibration test with the sensors onboard the satellite, were performed before the launch of TARANIS on 17 November 2020. The energy range of the LaBr3 crystals sensitive to X-rays and gamma rays was determined to be 0.04 to 11.6 MeV, 0.08 to 11.0 MeV, and 0.08 to 11.3 MeV for XGRE1, 2, and 3, respectively. The energy resolution at 0.662 MeV (full width at half maximum) was 20.5%, 25.9%, and 28.6%, respectively. The results from the calibration test were then used to validate a simulation model of XGRE and TARANIS. By performing Monte Carlo simulations with the verified model, we calculated effective areas of XGRE to X-rays, gamma rays, electrons, and detector responses to incident photons and electrons coming from various elevation and azimuth angles.