Inorganic glasses are promising candidates for scintillators because of their compositional flexibility, low cost, and ease of fabrication. However, their relatively low light yields and densities compared with those of crystal scintillators have limited their practical use, particularly for X-ray and γ-ray detection. In this study, P2O5–Al2O3–Cs2O–Gd2O3–Ce2O3 glasses were prepared by the melt-quenching method under reduced pressure, aiming to enhance both light yield and effective atomic number (Zeff). Ce³⁺ ions act as luminescent centers, whereas Gd³⁺ ions facilitate energy transfer and increase Zeff. Structural, photoluminescence (PL), and scintillation properties were investigated for glasses with various Ce/Gd molar ratios. Under X-ray excitation, the synthesized glasses exhibited a broad emission band centered at approximately 350 nm, attributed to the 5d–4f transition (2F7/2, 5/2) of Ce3+ ions. The highest light yield reached 2100 photons/MeV under 137Cs-γ-ray irradiation. The PL and scintillation decay profiles, as well as the temperature dependence of the emission spectra, indicate effective energy transfer from Gd3+ to Ce3+ ions. These results demonstrate that Ce³⁺/Gd³⁺ co-doped phosphate glasses are promising materials for efficient glass scintillators applicable to X-ray and γ-ray detection.