The GD library that RedHat installed was bundled with RH's PHP RPM and not a stand alone version. This is why you can't find it.
I typically don't install PHP and Apache with the OS and install the binary versions afterward so that I can easily reconfigure them as needed.
When configuring the PHP binary, --with-gd will tell it to use the bundled library and --with-gd=/location/to/gd will tell it to use a GD version that was installed separately.
For the easiest solution, check out the latest RPM version of PHP and see what GD version comes bundled with it. If it's the one you want, the easies thing to do is just update the RPM. I'm not sure if you are able to add configure options to and RPM but maybe someone else on this form with be able to help you.
If that doesn't work then I would uninstall the PHP and Apache RPMs and install the binaries being sure to include the --with-gd=/location/to/the/gd/lib/you/installed when configuring PHP.
If you want detailed instructions on how to install the binary versions, post away and I'll provide them. If someone has an RPM solution, I'd love to hear it because I've only used RPMs for programs that I know I will never have to modify.