GNOME has a couple of helpful methods for Logging out. You may either use the Logout menu item or the Logout button.
The Logout menu item is the first menu item you will find in the Main Menu. Simply select the Logout menu item and you will be prompted on whether you would like to log out or not. Select Yes and your GNOME session will end.
If you would like, you may add the Logout button to the Panel. This is just another method for logging out, it does not provide you any other functionality than the Logout menu item other than a pretty button. To add the Logout button to the Panel right mouse click on the Panel and select the Add logout button menu item from the pop-up menu. Once the button is there you may press it to log out.
NOTE: If you are running a window manager that is GNOME compliant the logout feature will quit the window manager as well as GNOME. If you are not running such a window manager you will have to end that window manager yourself.