A gas heat-pump system is provided. The gas heat-pump system may include an air-conditioning system including at least one compressor, an outdoor heat exchanger, an expander, an indoor heat exchanger, and a refrigerant pipe; an engine configured to provide power for an operation of the at least one compressor, and in which a mixed fuel, in which a fuel and air are mixed, is burned; a cooling water pump which pumps a flow of cooling water that cools the engine; a cooling water pipe connected to the cooling water pump, and configured to guide the flow of the cooling water; an auxiliary heat exchanger in which heat exchange between the cooling water flowing through the cooling water pipe and a refrigerant flowing through the refrigerant pipe is performed; a hot water heat exchanger, in which heat exchange between the cooling water flowing through the cooling water pipe and a fluid supplied from a hot water supply tank is performed; and a plurality of flow switches installed at the cooling water pipe, and controlled so that the cooling water discharged from the engine is guided to the auxiliary heat exchanger or the hot water heat exchanger.