A gateway is a network node that serves as an entry and exit point for a network connecting two networks with different sets of protocols, as well as converting one protocol into the other.

Difference between gateway and router

Gateway	Router
A device that acts as a gate and connects two networks having different sets of protocols.	A networking device that manages and forwards data packets to computer networks.
The main principle of a gateway is to convert one protocol to the other.	The main principle of the router is to route traffic from one network to another with the best route.
A gateway acts as a connection between two dissimilar networks.	A router transports data packets over similar networks.
A gateway does not support dynamic routing.	A router supports dynamic routing.
A gateway can operate until layer 5 of the OSI model.	A router can operate only on layer 3 and layer 4 of the OSI model.