You can use Command line for adding or changing the default gateway in Linux, follow the simple steps below.

Know the required command. The syntax is as follows:

• route add default gw {IP-ADDRESS} {INTERFACE-NAME}
• IP-ADDRESS=router IP Address INTERFACE-NAME= interface name such as eth0 (but may not be needed)
• Example: router IP address is 192.168.1.251

Login as root user:

• # route add default gw 192.168.1.251 eth0

Or use hostname such as home-router eth0

• # route add default gw home-router eth0