We have measured optical properties of GexGa4S96-x (x=22.5, 27, 30, 33.3 and 36) glasses including optical bandgap Eg, hardness, linear and nonlinear refractive index and laser damage threshold. We found that, both Eg and laser damage threshold exhibit maximum values in Ge30Ga4S66 glass, linear refractive index increases with increasing Ge content, but nonlinear refractive index has a minimum in Ge30Ga4S66, and their correlation can be well described by the Miller’s rule. We conclude that, Ge30Ga4S66 glass with chemically stoichiometric composition might be ideal for the chalcogenide-based optical amplifiers since it has reasonable optical nonlinearity, and high figure of merit (FOM) and laser damage threshold.