Relax, you havn't "completely destroyed" anything. The modular nature of linux means that you have just frozen gnome. You can always just use the x console, for eg.
If KDE is installed, then you can select it as a "session" in the login screen. There is usually a menu option for this. There are other session options to explore too.
In the absence of a session menu, you can always ctrl-alt-backspace into console mode, login from there. You can start kdm and/or edit your files from there.