Hi..esp for Redhat-9.0 users:
So redhat removed Window Maker from Redhat-9.0..now you might have installed Window Maker manually but gdmgreeter is not updated and it is still showing only KDE and GNOME there...you check /etc/X11/gdm/gdm.conf and find nothing there so that you can add your new Window Manager there..So what to do..
1.go to directory /usr/share/apps/switchdesk and you will see several Xclients.xxx files..so create a file there coresponding to your Window Manager for example...
# vi Xclients.wmaker
checkout other Xclients.xxx files for contents of new file...keep it similar to other files...and now make it executable
# chmod +x Xclients.wmaker
2.Now go to directory /etc/X11/xdm and edit file Xsession..you will find entries corresponding to GNOME,KDE there make a similar entry(similar line infact) for you Window Manager.
3.go to directory /etc/X11/gdm/Session and create a file corresponding to your Window manager and for the contents of this file checkout other files present in same directory with name KDE,GNOME,failsafe..etc.keep contents of new file similar to other.
make this file excutable
# vi wmaker
# chmod +x wmaker

and now restart gdm with command
and names of new Window Managers will be added to gdmgreeter you can select them and login...

PS:keep contents of new files created similar not identical...so i mean...for example file /usr/share/apps/switchdesk/Xclients.wmaker will look like...

exec wmaker