Gradient bandgap engineering has been acknowledged as an effective approach for improving the efficiency of CIGS solar cells. However, an effective method for achieving gradient bandgap engineering in quaternary sputtered CIGS films has not yet been identified.This is due to the uniform and singular nature of the quaternary target composition, as well as the complex kinetic mechanisms that result in the rearrangement of film elements (particularly Ga) during the annealing process. In this study, a grain boundary migration-driven diffusion mechanism for the Ga element in CIGS thin films was established to address the presented issue. The segregation of Ga element is believed to be caused by the flow velocity field generated from the migration of grain boundaries, and the distribution of Ga element can be precisely calculated using the mass transport equation. By modulating the V-shaped bandgap, a notable enhancement in open-circuit voltage (Voc) was achieved, with an increase from 0.405 V to 0.515 V, leading to an effectively improvement of solar cell's efficiency. By comparing and analyzing the actual performance parameters of the solar cells and the simulated results, we conclude that the increase in Voc is due to the alteration in bandgap structure. This study presents a novel approach to bandgap engineering in quaternary sputtered CIGS films through the quantitative calculation of Ga element distribution.