Dunst is a lightweight replacement for the notification daemons provided by most desktop environments. It’s very customizable, and isn’t dependent on any toolkits.
Review the USE flags
USE flags for x11-misc/dunst Customizable and lightweight notification-daemon
emerge --ask --verbose x11-misc/dunst
Unmerge other notification daemons
In order to avoid confusion other notification daemons could be removed, e.g. x11-misc/notification-daemon:
emerge --ask --verbose --depclean x11-misc/notification-daemon
D-BUS should start a notification daemon automatically, but if multiple are installed then it may just pick one. Starting dunst before any other notification daemons are fired up will make sure that dunst will handle your notifications. Review the used desktop setup on how to autostart programs.
After the installation you will have a sample configuration file /usr/share/dunst/dunstrc, which should be copied to ~/.config/dunst/dunstrc.
mkdir -p ~/.config/dunst
cp /usr/share/dunst/dunstrc ~/.config/dunst/
Customize ~/.config/dunst/dunstrc to your heart's content.
Test your notifications with notify-send:
notify-send "Summary" "Notification text."
Do not forget to kill dunst when you have changed your configuration file.