Gateway is used for differnt network.apart from gateway if we go from one network to another network so what we used?

From your  LAN you difine the routers IP address as the Gateway ( router is the boundary for your network), from yoyr  router you need to access the outside network, in that case you could apply two methods

1) use a default network  as the next network connected ( for Example Internet Service Provider)

  ip default-network NEXT network address

example ip default-network 172.20.165.0

you can add another instance for default network( this will be used if the above fails)

ip default-network192.168.100.0

2) even if you do not know the IP  address of the next hop router you can use the following

ip route0.0.0.00.0.0 . in this  case router will forward all traffic to the next  router and next  router will find the routes onwards

You need a router to communicate a different network. THen use a routing protocols to communicate two different networks