Installing Gnome on CentOS and make it launch at boot

First install Gnome (use ALT F2 to switch to the shell if needed and 'loadkeys fr' if you want to change the default keyboard locale) : yum groupinstall "GNOME Desktop Environment"

Alternatively, do a 'yum grouplist' to see the list of groups and run for example 'yum groupinstall Desktop'

Then run 'startx' to make sure it starts.
Then edit /etc/inittab and replace the last line with id:5:initdefault: