If an endpoint does not know how to reach its destination, it will send data to the specified default gateway. A default gateway is a device that routes traffic from a local network to other networks or the internet. The endpoint will use the default gateway's IP address as the next hop for packets that are destined for unknown or remote networks. The default gateway will then forward the packets to the appropriate destination or another gateway, based on its routing table. Reference:
Fundamentals of Network Security, Module 2: Networking Concepts, Lesson 2: IP Addressing and Routing1 PCCET Study Guide, Section 2.2: Describe IP Addressing and Routing2