You need to find a repo that offers a GD lib matching your current php version. I've had great success using Remi Collet's repo for this purpose. In fact, I used it yesterday to update my php install to the latest 5.4.0RC6 version on my CentOS6 box.
Once you've setup the repo it's a simple matter of running sudo yum install php-gd
Of course, the usual things apply: make sure you restart apache after installing, etc.