I think you're a bit mistaken, hope this helps.
You say for subnet
192.168.1.0/24, the default gateway is
192.168.1.1, it's one possibility. You can choose anything in the range of
192.168.1.1-255 as the default gateway.
A subnet is a logical grouping of connected network devices. Nodes on
a subnet tend to be located in close physical proximity to each other
on a LAN.
When a subnet is connected to the internet, most likely there is a router that links the LAN to the Internet. The default gateway is the IP address of the router interface connected to the LAN. Now let's say there are two routers in the LAN, both have an interface in the LAN with the same subnet and both are connected to the Internet. On a PC you can define which default gateway to use, seen as though both will send you to the Internet.