Lu3Al5-xGaxO12:Ce3+,Cr3+ powders are prepared using a solid-state reaction method. To determine the crystal structure, Rietveld refinement is performed. The results indicate that Ga3+ ions preferentially occupied tetrahedral rather than octahedral sites. The lattice constant linearly increases, obeying Vegard’s law, despite the strong preference of Ga3+ for the tetrahedral sites. Increasing x led to a blue-shift of the Ce3+ emission band in the green region and a change in the emission intensity. Persistent luminescence is observed from the powders prepared with x = 2–3, occurring through a trapping and detrapping process between Ce3+ and Cr3+ ions. The longest persistent luminescence is achieved for x = 2; its lifetime is at least 30 min. The findings are explained using crystal structure refinement, crystal field splitting, optical band gap, and electron trapping mechanism.