vi /etc/sysconfig/network GATEWAY=192.168.0.254
OR
vi /etc/sysconf/network-scripts/ifcfg-eth0 DEVICE=eth0
BOOTPROTO=static
ONBOOT=yes
IPADDR=192.168.0.?
NETMASK=255.255.255.0
GATEWAY=192.168.0.254
service network restart
Gateway defines the way to exit the packets. According to question System working as a router for two networks have IP Address 192.168.0.254 and 192.168.1.254.