Just started working with CentOS6 and came across an issue where I found it difficult to add a gateway for my eth0 interface. The netinstall I conducted didn’t appear to have an entry area for it (or I may have missed it). If you need to add or update your gateway do the following:

vi /etc/sysconfig/network
NETWORKING=yes
GATEWAY=192.168.0.254
/etc/init.d/network restart

Now your gateway should be working as expected.