The pressure-induced phase transition in amorphous germanium disulfide (a-GeS2) is studied using an ab initio constant-pressure technique. With the application of hydrostatic pressure, a-GeS2 undergoes a gradual phase transition from a semiconducting low-density amorphous state to a metallic high-density amorphous state. The transition is associated with a local coordination change in both the Ge atoms and S atoms. Upon pressure release, the high-density phase transforms back to a low-density amorphous state. The physical origin of the gradual phase transformation is discussed. The pressure-induced changes in the electronic and vibrational properties are studied with details. Additionally the pressure-induced phase transition of the monoclinic GeS2 is compared with that of amorphous state.