What is a default gateway? What happens if I don’t have one?

It  there is no gateway that means that the packet will not reach the other networks. so the gateway is more and more necessary 

The gateway is a routing device that knows how to pass traffic between different subnets and networks.

if there is no gateway that means that the packet will not reach the other networks.

Default gateway is the address of your router/ modem. It is common for all the devices connected to same network.

Without a default gateway you will not be able to route packets out of your LAN

default gateway is..0.0.1 and if you don't have one then there is defiantly an issue with your network card