If CentOS has already been installed without a graphical interface, you can install this with the following commands:
yum groupinstall "X Window System" "Desktop" "Desktop Platform"
yum install gdm
If you would like this to be the default runlevel, you can edit /etc/inittab and set the default level to 5 instead of 3:
vi /etc/inittab
Change:
id:3:initdefault:
To:
id:5:initdefault: