Documentation is too old
Manual currently says that it describes version 2.x of GDM and GConf-based settings. There is also many missing latest things such as theming, styling for GDM 3.x.
I do not know how and why GDM can be shows another appearance by editing some scripts such as css, some of distribution does not basically provide their own style-sheet file, which helps made easy to change style of GDM greeter screen. another number of some distributions handle their own filesystem for GDM and they distributions show it's own image and many color. many unofficial things should not let be it as is, it would better to be documented.
who someone can update obsolete manual for all of users, all of the world? I really would like to appreciate in advance.