Eu³⁺-doped gadolinium orthophosphate (GdPO₄) (Eu³⁺ at% = 0, 2, 5, 7,10,15, 20 and 30) nanoparticles have been prepared by ethylene glycol route and subsequently heated at 500 and 900 °C. The crystallite size increases with increasing heat-treatment temperature. Luminescence study shows that magnetic dipole transition (⁵D₀ ® ⁷F₁) is prominent over the electric dipole transition (⁵D₀ ® ⁷F₂), which has been attributed to occupancy of inversion symmetry site by more Eu³⁺ ions in Eu³⁺-doped GdPO₄. The luminescence intensity is enhanced as heat-treatment temperature increases from 500 to 900 °C due to the improved crystallinity. Optimum luminescence is observed for 5-7 at% Eu³⁺ in GdPO₄ nanoparticles. Above this concentration, luminescence intensity decreases due to concentration quenching effect. This is supported by lifetime study.