Turn Num Lock key on

This can be done through 2 steps.

1. Install "numlockx" 
2. Edit the /etc/gdm/Init/Default file

Install numlockx through terminal using this command
sudo yum install numlockx

Edit the /etc/gdm/Init/Default file through Gedit using this command in a terminal
sudo gedit /etc/gdm/Init/Default

In the file which opens, above exit 0, paste the lines below

if [ -x /usr/bin/numlockx ]; then
/usr/bin/numlockx on
fi

Save the file and exit.

Next time you reboot, num lock will be on by default

Gnome shortcut keys

Alt + F1: Open the Main Menu

Alt + F2: Open the Run Program dialog

F1: Display help

Alt + F4: Close the window

Alt + F10: Maximize the window

Alt + F5: Restore a maximized window to the original size of the window

Ctrl + F1: If the focused desktop element is a panel or application window, this keyboard shortcut switches the display of tooltips for the window or panel objects on and off

Shift + F10: Open a popup menu for the desktop element that has focus, if a popup menu exists

What is CentOS?

CentOS is an Enterprise-class Linux Distribution derived from sources freely provided to the public by a prominent North American Enterprise Linux vendor.

You can know more about CentOS at these sites
http://www.centos.org/
http://distrowatch.com/table.php?distribution=centos