The GDM configuration file ${datadir}/gdm/default.conf may be
replaced by your distribution on upgrade, so if a user wants
to ensure that configuration changes persist on upgrade, edit
the /etc/gdm/custom.conf file.  You can specify a different
location for the configuration files via the
"--with-defaults-conf=/path/to/file" and the
"--with-custom-conf=/path/to/file" options.  This can be
useful if you want to put the default file on a filesystem
shared by multiple machines.

If you wish to change configuration by hand, edit the
/etc/gdm/custom.conf file and make sure the keyname=value pair
you want is included in the appropriate section.  For example,
to change the "Greeter" key in the "daemon" section, make sure the
daemon section of the custom.conf file has the value like
in this example.  The modified option does not have to come
first after the "[daemon]" section heading, but can be placed
anywhere in the section.