Same thing here, /etc/gdm.conf was present but empty. I've repaired it by copying /usr/share/gdm/defaults.conf over /etc/gdm/gdm.conf. After doing this I can change GDM settings using gdmsetup. It's probably Debian-specific.
-- System Information: Debian Release: lenny/sid APT prefers testing APT policy: (500, 'testing') Architecture: i386 (i686) Kernel: Linux 2.6.27.5 (SMP w/2 CPU cores; PREEMPT) Locale: LANG=pl_PL.UTF-8, LC_CTYPE=pl_PL.UTF-8 (charmap=UTF-8) Shell: /bin/sh linked to /bin/dash Versions of packages gdm depends on: ii adduser 3.110 add and remove users and groups ii debconf [debconf-2.0] 1.5.22 Debian configuration management sy ii gksu 2.0.0-5 graphical frontend to su ii gnome-session [x-sessi 2.22.3-2 The GNOME 2 Session Manager ii gnome-terminal [x-term 2.22.3-3 The GNOME 2 terminal emulator appl ii libart-2.0-2 2.3.20-2 Library of functions for 2D graphi ii libatk1.0-0 1.22.0-1 The ATK accessibility toolkit ii libattr1 1:2.4.43-1 Extended attribute shared library ii libc6 2.7-15 GNU C Library: Shared libraries ii libcairo2 1.6.4-6.1 The Cairo 2D vector graphics libra ii libdbus-1-3 1.2.1-4 simple interprocess messaging syst ii libdbus-glib-1-2 0.76-1 simple interprocess messaging syst ii libdmx1 1:1.0.2-3 X11 Distributed Multihead extensio ii libfontconfig1 2.6.0-1 generic font configuration library ii libfreetype6 2.3.7-2 FreeType 2 font engine, shared lib ii libglade2-0 1:2.6.2-1 library to load .glade files at ru ii libglib2.0-0 2.16.6-1 The GLib library of C routines ii libgnomecanvas2-0 2.20.1.1-1 A powerful object-oriented display ii libgtk2.0-0 2.12.11-4 The GTK+ graphical user interface ii libpam-modules 1.0.1-4 Pluggable Authentication Modules f ii libpam-runtime 1.0.1-4 Runtime support for the PAM librar ii libpam0g 1.0.1-4 Pluggable Authentication Modules l ii libpango1.0-0 1.20.5-3 Layout and rendering of internatio ii librsvg2-2 2.22.2-2 SAX-based renderer library for SVG ii librsvg2-common 2.22.2-2 SAX-based renderer library for SVG ii libselinux1 2.0.65-5 SELinux shared libraries ii libwrap0 7.6.q-16 Wietse Venema's TCP wrappers libra ii libx11-6 2:1.1.5-2 X11 client-side library ii libxau6 1:1.0.3-3 X11 authorisation library ii libxdmcp6 1:1.0.2-3 X11 Display Manager Control Protoc ii libxext6 2:1.0.4-1 X11 miscellaneous extension librar ii libxi6 2:1.1.3-1 X11 Input extension library ii libxinerama1 2:1.0.3-2 X11 Xinerama extension library ii libxml2 2.6.32.dfsg-4 GNOME XML library ii lsb-base 3.2-20 Linux Standard Base 3.2 init scrip ii metacity [x-window-man 1:2.22.0-2 A lightweight GTK2 based Window Ma ii rxvt-unicode [x-termin 9.05-1+lenny1 RXVT-like terminal emulator with U ii zlib1g 1:1.2.3.3.dfsg-12 compression library - runtime Versions of packages gdm recommends: ii dialog 1.1-20080316-1 Displays user-friendly dialog boxe ii gdm-themes 0.6.1 Themes for the GNOME Display Manag ii whiptail 0.52.2-11.3 Displays user-friendly dialog boxe ii xnest 2:1.4.2-7 Nested X server ii xserver-xephyr 2:1.4.2-7 nested X server ii xserver-xorg 1:7.3+18 the X.Org X server ii zenity 2.22.1-2 Display graphical dialog boxes fro Versions of packages gdm suggests: ii libpam-gnome-keyring 2.22.3-2 PAM module to unlock the GNOME key ii locales 2.7-15 GNU C Library: National Language ( ii pm-utils 1.1.2.4-1 utilities and scripts for power ma -- debconf information: gdm/daemon_name: /usr/bin/gdm * shared/default-x-display-manager: gdm -- Bartek Krawczyk JID: [EMAIL PROTECTED] -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of "unsubscribe". Trouble? Contact [EMAIL PROTECTED]