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 again.