We have developed a hot-pressing method for consolidating garnet scintillator powder of composition Gd3(Al,Ga)5O12:Ce. Thermodynamic analysis revealed that gallium oxide, which was a constituent of the garnet scintillator powder, would react with the graphite mold beginning at 950°C and would be reduced to gallium metal. To prevent this reaction during hot-pressing, a reaction barrier material was placed between the graphite mold and the scintillator powder. As for the reaction barrier material, 3GdAlO3Al2O3 powder calcined at 1100°C was found to be suitable because the material provided two important characteristics: the same thermal expansion coefficient as the scintillator material and good sinterability. The Gd3(Al,Ga)5O12:Ce garnet scintillator powder was hot pressed at 1475°C for 4 h in vacuum, followed by annealing at 1300°C for 4 h in oxygen atmosphere. The thus obtained garnet scintillator successfully satisfied the necessary requirements for use in an X-ray computed tomography detector.