I created 4 routers basically 2 router for default route with interface (g0/0) and the other 2 is defaulted by the ip interface and they did the same purpose. Im wondering what is the different between them.
Quoting from Cisco documentation:
If you point a static route to an interface and do not specify the next hop IP address, the route is inserted into the routing table only when the interface is active. This configuration is not recommended because when the static route points to an interface and has no next hop information, the router considers each of the hosts within the range of the route to be directly connected through that interface. An example of such a static route is ip route 0.0.0.0 0.0.0.0 Ethernet0.
With this type of configuration, a router performs Address Resolution Protocol (ARP) on the Ethernet for every destination that the router finds through the default route because the router considers all of these destinations as directly connected to Ethernet 0. This kind of static route, especially if it is used by many packets to many different destination subnets, can cause high processor utilization and a very large ARP cache (along with memory allocation failures). Therefore, this kind of static route is not recommended.
When you specify the next hop address on a directly connected interface, the router does not perform ARP for each destination address. An example is ip route 0.0.0.0 0.0.0.0 Ethernet0 192.168.1.1.