We report a study of the roles of gadolinium(III) (Gd³⁺) dopants in influencing the catalytic activity of gadolinium-doped ceria nanoparticles towards the pro-oxidation of hydrogen peroxide to hydroxyl radicals. These doped ceria nanoparticles with dopant concentrations of 0.6 wt.%, 3 wt.%, and 6 wt.% Gd³⁺ were synthesized using an ozone-mediated method for tuning their catalytic activities. The Gd dopants were found to foster an increase in the percentage of Ce³⁺ ions in the doped ceria nanoparticles. Our reaction kinetic study revealed that the relationship between the overall reaction rates and the Gd dopant concentrations in our doped materials followed a volcano-like trend. In contrast, the apparent activation energy values of these Gd-doped ceria nanoparticles were found to be positively associated with the concentrations of Gd dopants. The overall catalytic activity trend was attributed to the interplay between the promotion and degradation effects of the Gd dopants on the properties of doped ceria nanoparticles.