A gas heat pump system comprising: an outdoor unit comprising a compressor (110, 112), an outdoor heat exchanger, and an expansion device; an indoor unit comprising an indoor heat exchanger; a refrigerant pipe configured to connect the outdoor unit and the indoor unit so as to allow a refrigerant to circulate through the outdoor unit and the indoor unit, wherein the refrigerant pipe comprises a suction pipe that is configured to guide suction of the refrigerant to the compressor (110,112) and a discharge pipe that is configured to guide discharge of the refrigerant from the compressor (110,112); an engine configured to combust mixed fuel in which fuel and air are mixed so as to provide power for driving the compressor (110, 112); a coolant tank configured to store a coolant for cooling the engine; a coolant pump configured to allow the coolant stored in the coolant tank to forcibly flow; a radiator configured to emit, to an outside, heat which is transferred from the engine to the coolant; and a coolant pipe configured to connect the coolant tank, the coolant pump, and the radiator so as to allow the coolant to circulate therethrough, wherein the gas heat pump system has a cooling capability in the range of 71 kW to 85 kW, a refrigerant, wherein a mixed refrigerant containing R32 of 50% or more is used as the refrigerant, characterised in that the refrigerant pipe comprises a ductile stainless steel pipe, wherein the stainless steel material comprises, percent by weight: C: 0.03% or less; Si: more than 0% and less than 1.7%; Mn: 1.5% to 3.5%; Cr: 15% to 18%; Ni: 7.0% to 9.0%; Cu: 1.0% to 4.0%; Mo: 0.03% or less; P: 0.04% or less; S: 0.04% or less; and N: 0.03% or less; wherein the stainless steel has an austenite matrix structure of 99% or more, and a delta ferrite matrix structure of 1% or less, on the basis of a grain area; and an average grain diameter of the austenite matrix structure is 30 μm to 60 μm; wherein the suction pipe has an outer diameter of 25.40 mm and an inner diameter of 24.10 mm or less; and wherein the discharge pipe has an outer diameter of 19.05 mm and an inner diameter of 18.07 mm or less. an outdoor unit comprising a compressor (110, 112), an outdoor heat exchanger, and an expansion device; an indoor unit comprising an indoor heat exchanger; a refrigerant pipe configured to connect the outdoor unit and the indoor unit so as to allow a refrigerant to circulate through the outdoor unit and the indoor unit, wherein the refrigerant pipe comprises a suction pipe that is configured to guide suction of the refrigerant to the compressor (110,112) and a discharge pipe that is configured to guide discharge of the refrigerant from the compressor (110,112); an engine configured to combust mixed fuel in which fuel and air are mixed so as to provide power for driving the compressor (110, 112); a coolant tank configured to store a coolant for cooling the engine; a coolant pump configured to allow the coolant stored in the coolant tank to forcibly flow; a radiator configured to emit, to an outside, heat which is transferred from the engine to the coolant; and a coolant pipe configured to connect the coolant tank, the coolant pump, and the radiator so as to allow the coolant to circulate therethrough, wherein the gas heat pump system has a cooling capability in the range of 71 kW to 85 kW, a refrigerant, wherein a mixed refrigerant containing R32 of 50% or more is used as the refrigerant, characterised in that the refrigerant pipe comprises a ductile stainless steel pipe, wherein the stainless steel material comprises, percent by weight: C: 0.03% or less; Si: more than 0% and less than 1.7%; Mn: 1.5% to 3.5%; Cr: 15% to 18%; Ni: 7.0% to 9.0%; Cu: 1.0% to 4.0%; Mo: 0.03% or less; P: 0.04% or less; S: 0.04% or less; and N: 0.03% or less; C: 0.03% or less; Si: more than 0% and less than 1.7%; Mn: 1.5% to 3.5%; Cr: 15% to 18%; Ni: 7.0% to 9.0%; Cu: 1.0% to 4.0%; Mo: 0.03% or less; P: 0.04% or less; S: 0.04% or less; and N: 0.03% or less; wherein the stainless steel has an austenite matrix structure of 99% or more, and a delta ferrite matrix structure of 1% or less, on the basis of a grain area; and an average grain diameter of the austenite matrix structure is 30 μm to 60 μm; wherein the suction pipe has an outer diameter of 25.40 mm and an inner diameter of 24.10 mm or less; and wherein the discharge pipe has an outer diameter of 19.05 mm and an inner diameter of 18.07 mm or less.