We synthesized Ce-doped Gd3Al5−yGayO12 (GAGG) nanoparticle scintillators for obtaining efficient emission in living bodies upon X-ray irradiation from outside. To obtain a high photoluminescence quantum yield (QY), the chemical composition and synthesis conditions were optimized. The highest QY of 90%, which is higher than that of a commercially available Ce-doped GAGG single-crystal scintillator (87%), was achieved with the composition for Gd:Al:Ga = 3:3:2, a Ce concentration of 0.1 mol% relative to (Gd + Ce), aging at room temperature for 1 d, and a calcination temperature of 1300 °C. The size of the particles was 200–300 nm. A dominant emission band at around 550 nm was observed in the photoluminescence and X-ray-induced radioluminescence spectra and attributed to the 5d–4f transition of Ce3+. The high photoluminescence QY and appropriate size for applications in living bodies indicated that nanoparticle scintillators based on Ce-doped GAGG were successfully developed.