a network gateway can also be a "Bridge" which "routes" (here it is ) two networks (its a node between two different virtual networks for example.
btw. i have a Bridge at home running Debian 4.0 - which doesnt route a network. it has my self written small firewall which mainly filters every unwanted IPs/or Ranges, like IPs from the Industry or something like this....but it isnt a Firewall, its just a Invisible PC which filters IPs and Packages invisibly