Installing the GD library is easy on CentOS thanks to yum.  Simply type:

yum install gd gd-devel php-gd

And all your GD fun is installed and ready to go.  I was trying to install the Perl module GD::Graph and it wouldn’t compile, complaining:

GD.xs:7:16: error: gd.h: No such file or directory

After installing the GD-Devel package, everything is fine [...]