The series of Gd3-xCexGa3Al2O12 nanopowders doped with different concentrations of Ce3+ ions were prepared by Pechini (sol-gel) and combustion methods. The structure and morphology of the powders were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. It was found that the synthesis method has a great impact on the morphology and, consequently, spectroscopic properties of the powders. Optical properties of the powders were examined using excitation, emission, and luminescence kinetic measurements. For all powders, persistent luminescence and emission decay processes were studied. The most intense luminescence was observed for the powder with 0.5 mol % of Ce3+ synthesized using the combustion method and 1 mol % in the case of the sol-gel sample. The longest and brightest persistent luminescence was observed for the powders doped with 0.1 mol % (combustion) and 0.2 mol % of Ce3+ ions (sol-gel). The thermoluminescence measurements were done for the powders prepared using different methods to understand the impact of the synthesis conditions on the number and depths of the traps involved in persistent luminescence. On the basis of spectroscopic measurements, the mechanism of persistent luminescence was constructed and discussed.