Gateways regulate traffic between two dissimilar networks. In the case of the SmartNodes, this means from Analog-IP (SIP) or ISDN-IP (SIP). These devices will only need to have one Ethernet port as they will not be doing any IP routing.
Gateway/Routers will also route between two dissimilar network types, but they also regulate traffic between similar networks, in this case the TCP/IP network. These devices will need to have at least two Ethernet ports so that the IP traffic can be routed from one network to another.
Most companies already have a router in place for the IP traffic, so they will need only the Gateway feature to convert their old Telephony equipment to IP (SIP) and vice versa.