A gas engine system comprising: a gas engine; an exhaust pipe connected to the gas engine, the exhaust pipe being a pipe through which exhaust gas discharged from the gas engine flows; a turbocharger connected to the exhaust pipe; a catalyst disposed in a passage extending from the exhaust pipe to the turbocharger, the catalyst oxidizing uncombusted fuel gas in the exhaust gas discharged from the gas engine; a catalyst temperature detector configured to measure a temperature of the catalyst; a catalyst activating device configured to supply an activation promoting agent that promotes activation of the catalyst to a passage extending from the exhaust pipe to the catalyst; and a controller including a first functional block configured to operate the catalyst activating device to supply the activation promoting agent when the temperature of the catalyst measured by the catalyst temperature detector is lower than a predetermined threshold, characterized by: an oxygen concentration detector configured to measure an oxygen concentration in the exhaust gas of the gas engine at a position upstream of the catalyst; and an intake air amount detector configured to measure an intake air amount of the gas engine, wherein the activation promoting agent includes a first activation promoting agent, the first activation promoting agent containing a hydrocarbon that has a less activation energy value for a catalytic reaction catalyzed by the catalyst than the fuel gas of the gas engine, and wherein the controller includes a third functional block configured to, when the temperature of the catalyst measured by the catalyst temperature detector is lower than the predetermined threshold, adjust at least one of the intake air amount of the gas engine, an oxygen concentration in the intake air, and a fuel injection amount of the gas engine based on the oxygen concentration measured by the oxygen concentration detector and the intake air amount measured by the intake air amount detector to decrease an excess air ratio of the gas engine from the excess air ratio intended for normal operation. a gas engine; an exhaust pipe connected to the gas engine, the exhaust pipe being a pipe through which exhaust gas discharged from the gas engine flows; a turbocharger connected to the exhaust pipe; a catalyst disposed in a passage extending from the exhaust pipe to the turbocharger, the catalyst oxidizing uncombusted fuel gas in the exhaust gas discharged from the gas engine; a catalyst temperature detector configured to measure a temperature of the catalyst; a catalyst activating device configured to supply an activation promoting agent that promotes activation of the catalyst to a passage extending from the exhaust pipe to the catalyst; and a controller including a first functional block configured to operate the catalyst activating device to supply the activation promoting agent when the temperature of the catalyst measured by the catalyst temperature detector is lower than a predetermined threshold, characterized by: an oxygen concentration detector configured to measure an oxygen concentration in the exhaust gas of the gas engine at a position upstream of the catalyst; and an intake air amount detector configured to measure an intake air amount of the gas engine, wherein the activation promoting agent includes a first activation promoting agent, the first activation promoting agent containing a hydrocarbon that has a less activation energy value for a catalytic reaction catalyzed by the catalyst than the fuel gas of the gas engine, and wherein the controller includes a third functional block configured to, when the temperature of the catalyst measured by the catalyst temperature detector is lower than the predetermined threshold, adjust at least one of the intake air amount of the gas engine, an oxygen concentration in the intake air, and a fuel injection amount of the gas engine based on the oxygen concentration measured by the oxygen concentration detector and the intake air amount measured by the intake air amount detector to decrease an excess air ratio of the gas engine from the excess air ratio intended for normal operation. an oxygen concentration detector configured to measure an oxygen concentration in the exhaust gas of the gas engine at a position upstream of the catalyst; and an intake air amount detector configured to measure an intake air amount of the gas engine, wherein the activation promoting agent includes a first activation promoting agent, the first activation promoting agent containing a hydrocarbon that has a less activation energy value for a catalytic reaction catalyzed by the catalyst than the fuel gas of the gas engine, and wherein the controller includes a third functional block configured to, when the temperature of the catalyst measured by the catalyst temperature detector is lower than the predetermined threshold, adjust at least one of the intake air amount of the gas engine, an oxygen concentration in the intake air, and a fuel injection amount of the gas engine based on the oxygen concentration measured by the oxygen concentration detector and the intake air amount measured by the intake air amount detector to decrease an excess air ratio of the gas engine from the excess air ratio intended for normal operation.