you can't log in as root in Gnome anymore (unless you go through and change some system files to allow it.)
But there really is no reason that you need to log in as root in Gnome, There are numerous ways to do waht you need without actually logging in as root in a GUI.
1: open a terminal, su to root, then run yum to install the packages you want.
2: ctrl-alt-F2 to get to a console screen, sign in as root, run yum to install the packages.
3: open a terminal, su to root, run yum to install yumex, which is a graphical front-end to yum.
yum install yumex
Then go to your Applications overview.. Activities-->Applications-->System Tools and click yumex. The enter root password and it will give you a graphical environment to select packages and install.
4: run packagekit, and when it asks for authentication, use your user (not root) password (if the user has been set up as administrative user).