It used the be the case in previous versions of CentOS that one could install a gnome desktop or kde desktop by using yum groupinstall.
However, in centos 6 it is generalized. To install a desktop one can still use the groupinstall, however it is now generic. To install it do:
yum groupinstall 'Desktop'
Make sure to edit /etc/inittab and change the runlevel to 5, otherwise the desktop environment will not be brought up.
cat /etc/inittab ... id:5:initdefault:
Then reboot your centos 6 host for the settings to take effect.