Using the Linux route command

Add a default gateway

route add default gw 10.0.0.1

This will add a temporary route that will clear after reboot.  To set the
default gateway betwen reboots edit /etc/sysconfig/network with the following
line

GATEWAY=10.0.0.1

Add a static route

route add -net 192.168.100.0 netmask 255.255.255.0 eth0

This will route all traffic toward this subnet to the eth0 interface

Manually add a static non gateway route

route add -net 10.8.100.0 netmask 255.255.255.0 gw 10.0.16.140

you can add stuff to the file /etc/sysconfig/static-routes
to make stuff start on boot

The syntax is a bit different than the route command.  Check the file
/etc/sysconfig/network-scripts/ifup-route to see the syntax it's using.