Constructing Type-II heterostructure is an effective scheme to tailor the electronic structure and improve the application performance. Motivated by recently successful syntheses of Mg(OH)₂ and GaS monolayers, we investigate the stability, electronic, and optical properties of GaS/Mg(OH)₂ heterostructure by using the density functional theory method. The calculated results show that GaS/Mg(OH)₂ heterostructure is easily constructed due to its small lattice mismatch, negative binding energy, and thermodynamic stability. Compared with monolayer materials, the GaS/Mg(OH)₂ heterostructure has a band gap that effectively decreases to 2.021 eV and has Type-II band structure, facilitating the spatial separation of photo-generated carriers where electrons are localized in the GaS and holes reside in the Mg(OH)₂ monolayers. The built-in electric field induced by the interlayer charge transfer points from GaS to Mg(OH)₂ monolayer, which can further improve the separation and suppress the recombination of electron-hole pairs. Under the biaxial strain, the valance band maximum and conduction band minimum of GaS/Mg(OH)₂ heterostructure shift in the downward direction to different extents, resulting in obvious change of band gap, with the change reaching about 0.5 eV. Furthermore, the band structure of GaS/Mg(OH)₂ heterostructure can be transformed from indirect band gap semiconductor into direct band gap semiconductor under the tensile strain, while GaS/Mg(OH)₂ heterostructure maintains Type-II band structure. Additionally, the band edge positions of GaS/Mg(OH)₂ heterostructure can also be effectively adjusted to cross the redox potentials of water decomposition at pH = 0–7. The light absorption spectra show that GaS/Mg(OH)₂ heterostructure has stronger light absorption capability than the constituent monolayers. Especially, the light absorption has an obvious redshift phenomenon at a tensile strain of 3%. These findings indicate that the GaS/Mg(OH)₂ heterostructure has a wide range of applications in the field of optoelectronics due to the tunable electronic properties, and also provides some valuable insights for future research.