The development of efficient optical limiting materials has gained considerable attention due to their critical role in safeguarding human eyes, skin and sensitive optoelectronics from harmful class 3B laser exposure. Pristine and doped GdPO4 based nanostructures have not been explored for (nonlinear optical) NLO application. Herein, we explore a Sm3+/Eu3+ co-doped gadolinium (Gd) based orthophosphate for NLO based on the two photon absorption (2PA) mechanism. The influence of Sm3+/Eu3+ co-doping on the transition from genuine to sequential 2PA in GdPO4 nanostructures is systematically investigated for optical limiting (OL) applications. Urbach energy calculations revealed co-doping increases the density of defect states. A prolonged excited-state lifetime in doped samples suggests the availability of excited carriers, facilitating excited-state absorption (ESA). Computational density of state (DOS) calculations confirm dopant-induced localized impurity states in the host lattice. The nonlinear optical absorption (NLA) is explored using intensity-dependent open aperture Z-scan measurements under 532 nm nanosecond laser excitation, exhibiting a transition from genuine 2PA in pristine samples to sequential 2PA in doped samples. The nonlinear absorption coefficient (β) of GdPO4 is reported to be in the order of 10−10 m W−1; the highest value for orthophosphate. These findings highlight the potential of co-doped GdPO4 as a promising candidate for advanced OL and laser protection applications.