The gdm.conf file format is not included in the rpm.
It is very necessary to have the documentation included to setup custom X
configurations. The html files under the docs directory in the source
would be great.
As far as I can tell the RPM includes all docs. the HTML help is in
/usr/share/gnome/help/gdm or something though, it's intended to be found from
the GUI help browser.
You might also try the 'gdmconfig' app to configure gdm, instead of editing the
gdm.conf file. Some people prefer the GUI config tool.