I think if you know the pid of the gnome session manager, then you read the environment from /proc filesystem.
GNOME_SESSION_PID=<PID_OF_GNOME_SESSION>
READ_SESSION_COOKIE="$(grep -z DBUS_SESSION_BUS_ADDRESS /proc/$pid/environ|cut -d= -f2-)"
Use the SESSION id then, with other programs like notify-send or dbus* tools.
Cheers.