From：https://www.zhihu.com/question/21787311?from=profile_question_card
Gateway:
A gateway is a concept and does not refer to a specific device; as long as a device connecting two different networks can be called a gateway.
router:
A device that can realize route finding and forwarding.
The router is obviously capable of fulfilling the function of a gateway.
Default gateway:
The concept of a network layer. The PC does not have the function of routing and forwarding, so the PC should first send all packets to a default transit device for forwarding, that is, the default gateway.
This gateway can be on routers, Layer 3 switches, firewalls, servers and other devices.