Gateways (routers) do not necessarily run any NAT variant (you do not NAT on routers internal to your own network; use NAT for private<->public or overlapping addressing, but NAT is not a substitute for routing). Routers are just hosts on the LAN that know how to reach other networks, so they can be assigned any valid host address on the network.
Some people always assign the first usable host address, some always assign the last usable host address, and some assign any usable host address. It does not matter which usable host address, but it is a best practice to pick something and consistently do it the same way every time you assign a router address.