Monolayer Gallium sulfide (GaS) was demostrated as promising two-dimensional semiconducting material with considerable band gap. The present work, investigates the band gap modulation of monolayer GaS under biaxial or uniaxial strain by using Density functional theory calculation. We found that monolayer GaS shows an indirect band gap that limits its optical applications. The results show that monolayer GaS has sizable band gap. Under uniaxial strain, a unique behavior of GaS is observed. It shifts from indirect to direct band gap semiconductor. This behavior, allowing applications such as electroluminescent devices and lasre. The detailed reasons for the band gap modulation are also discussed by analyzing projected density of states (PDOS). It indicate that, due to the role of py orbital through uniaxial strain become more significant than others near the Fermi level. The indirect to direct band gap transition happen at "=-10y%. Moreover by investigating the strain energy and transverse response of structure under uniaxial strain, we show that, the monolayer GaS has poission’s ratio of 0.23 and 0.24 in zigzag (x) and armchair (y) directions, respectively. Thus, we conclude that the isotropic nature of mechanical properties under strain.
