A gateway apparatus and a message routing method are provided. The gateway apparatus includes: a network adaptor transmitting and receiving a message through a vehicle network or a diagnosing CAN positioned inside a vehicle; a CAN driver transferring the message received through the network adaptor; a message router routing the message transferred from the CAN driver and filtering valid data of the message; an application invoking a signal routing application to detect validity of data of each signal of the message transferred from the CAN driver and route data of a valid signal; and a CAN diagnostor processing a message of a diagnosing apparatus performing an access through the diagnosing CAN positioned inside the vehicle to control a diagnosing operation.