A gateway device, connected to one or more buses used in communication by a plurality of ECUs on-board a vehicle, is provided with: a receiving unit that receives, from a server that acts as an external device external to the vehicle, firmware update information that includes updated firmware to be applied to one ECU from among the plurality of ECUs; and a control unit that determines, based on certain information about the ECU on which to apply the updated firmware, whether or not the ECU satisfies a certain condition, and if the certain condition is satisfied, causes the ECU to execute a certain process related to updating firmware, whereas if the certain condition is not satisfied, causes equipment other than the ECU to execute the certain process.