Hydrogen storage properties of co-functionalized 2D GaS monolayer have been systematically investigated by first-principles calculations. The strength of the binding energy of hydrogen (H-2) molecules to the pristine GaS surface shows the physisorption interactions. Co-functionalized GaS sheet by Li, Na, K and Ca atoms enhanced the capacity of binding energies of hydrogen and strength of hydrogen storage considerably. Besides, DFT calculations show that there is no structural deformation during H-2 desorption from cofunctionalized GaS surface. The binding energies of per H-2 molecules is found to be 0.077 eV for pristine GaS surface and 0.064 eV-0.37 eV with the co-functionalization of GaS surface. Additionally, in the presence of applied external electric field enhanced the strength of binding energies and it is found to be 0.09 eV/H-2 for pristine GaS case and 0.19 eV/H-2 to 0.38 eV/H-2 for co-functionalized GaS surface. Among the studied GaS monolayer is found to be the superior candidate for hydrogen storage purposes. The theoretical studies suggest that the electronic properties of the 2D GaS monolayer show the electrostatic behavior of hydrogen molecules which confirms by the interactions between adatoms and hydrogen molecules before and after hydrogen adsorption. (C) 2020 The Author(s). Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.