What version of linux are you using? Ubuntu? CentOS?
This will help me determine the configuration files that you are currently using.
In a debian-based environment, there is the configuration file /etc/network/interfaces.
In this case, we would typically use the following syntax:
iface eth0 inet static
up route add default gw 192.168.0.1
This would specify that for eth0 the default gateway is 192.168.0.1. This would be a permanent route.
Hope this helps!!