I'm using Archlinux. I've recently switched back from XFCE 4 to Gnome 3. So i reinstalled
notification-daemon (since there was a conflict between it and XFCE 4 notification daemon)
pacman -S notification-daemon
.. and removed
pacman -R xfce4-notifyd
But now, notification in Gnome 3 is not working at all. I don't know what else should i do.