A gateway device for a vehicle network system is provided. The vehicle network system includes a first network, a second network, a first electronic control unit connected to the first network, a second electronic control unit connected to the second network, and the gateway device connected to the first network and the second network. The gateway device receives a first frame transmitted to the first network by the first electronic control unit; determines whether or not the first frame is appropriate; generates a second frame when the first frame is not determined to be appropriate; and transmits the second frame to the second network. The second frame includes control information and additional information based on content of the first frame. The control information restricts processing of the additional information included in the second frame by the second electronic control unit.