Proposed is a gas heat-pump system including: a compressor discharging compressed refrigerant; an indoor heat exchanger causing heat exchange to occur between indoor air and the refrigerant; an outdoor heat exchanger condensing the refrigerant; a four-way valve switching a flow direction; an engine; a radiator cooling heated coolant; an exhaust gas heat exchanger causing the heat exchange to occur between the coolant passing through the radiator and exhaust gas from the engine; and a first three-way valve switching a flow direction of the coolant and controlling an amount of the flowing coolant in such a manner that the coolant passing through the radiator flows toward at least one of the exhaust gas heat exchanger and the engine.