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