A gas heat pump system including an outdoor unit having a compressor, an outdoor heat exchanger, and an expansion device; an indoor unit having an indoor heat exchanger; a refrigerant pipe to connect the outdoor unit and the indoor unit; an engine to combust mixed fuel in which fuel and air are mixed; a coolant tank to store a coolant; a radiator to emit, to an outside, heat which is transferred from the engine to the coolant; and a coolant pipe to connect the coolant tank and the radiator to allow the coolant to circulate therethrough, whereby the gas heat pump system has a cooling capability between 71 kW and 85 kW, the refrigerant is a mixed refrigerant having at least 50% R32, and the refrigerant pipe is a ductile stainless steel pipe having a delta ferrite matrix structure of 1% or less based on grain area.