The controlled synthesis of large and uniform gallium selenide (GaSe) crystals is crucial for its various applications based on the attractive properties of this emerging material. In this work, vapor phase growth of high-quality monolayer GaSe nanosheets with multiple shape and size is achieved by tuning the Ga/GaSe ratio in the precursor. A theoretical model based on density functional theory calculations and kinetic Wulff construction theory describe the observed shape evolution of the GaSe nanosheets. Results show that the Ga/Se ratio plays a critical role in the evolution of the domain shape and size. Moreover, the as-grown GaSe nanosheets show improved performance with photoresponse time less than 0.7 ms and responsibility up to 3,000 A/W. This study presents a previously unexplored strategy for the controlled growth of two-dimensional (2D) GaSe nanosheets for promising applications in next-generation optoelectronics.