The present invention is to provide a gateway device which, when determining that a network device connected thereto is a web camera and receives a packet from the web camera, is able to automatically add a port of the gateway device connected with the network device to a VLAN designated by the gateway device and then add a specific group code to the packet to specify the VLAN, such that the gateway device can transmit the packet to the VLAN according to the specific group code. The gateway device is then able to automatically set transmission priority of the packet to a high priority, such that after the packet is transmitted out of the VLAN, the packet can be transmitted through a transmission channel with the high priority, so as to improve the convenience of using the web camera and assure the transmission priority of the packet in the VLAN.