I am assuming that (a) you wish to switch desktops temporarily, and (b) you have KDE installed. So, logout ("End Session") from GNOME, and at the login screen, find the "Session" Menu, click on KDE, and login again as normal. Note that you will have to configure all of the KDE-specific desktop applications (e.g., Konqueror file manager, panels, wallpaper, screensaver, etc), but not any of the general applications.
If you do not have KDE desktop installed, open a terminal as root user and execute:
yum groupinstall KDE (K Desktop Environment)
exactly like that. Then proceed as above.