Right now, we have the following components running on PureDarwin 9.
Metacity is the window manager most commonly used with GNOME. Once you have installed it (e.g., from MacPorts) and try to launch it, you will get
Window manager warning: Failed to contact configuration server; some possible causes are that you need to enable TCP/IP networking for ORBit, or you have stale NFS locks due to a system crash. See http://projects.gnome.org/gconf/ for information. (Details - 1: Failed to get connection to session: Not enough memory) Dynamic session lookup supported but failed: launchd did not provide a socket path, verify that org.freedesktop.dbus-session.plist is loaded! Window manager warning: Failed to contact configuration server; some possible causes are that you need to enable TCP/IP networking for ORBit, or you have stale NFS locks due to a system crash. See http://projects.gnome.org/gconf/ for information. (Details - 1: Failed to get connection to session: Not enough memory)
Then simply
launchctl load -w /Library/LaunchAgents/org.freedesktop.dbus-session.plist metacity
and it will run.
Zenity is a little tool that can be used (e.g., in bash scripts) to create simple GTK dialogs.