From FC-2 onwards they have changed the way Window manager sessions are loaded by GDM and KDM.So if you have installed some new window manager from the source and want to use it through GDM menu..read on...

For compatiblity issues GDM and KDM,both now read the window manger sessions from "/etc/X11/dm/Sessions/".If you do a "cd" to that directory you will find entries such as "gnome.desktop","KDE.desktop"..thus make a similar entry for your favourite window manager.And you restart gdm using command "gdm-restart".

And now you can select your window manager from the GDM menu.