$ sudo apt-get update
$ sudo apt-get install gnome-gmail-notifier 

gnome-gmail-notifier is installed in your system.

Make ensure the gnome-gmail-notifier package were installed using the commands given below,

$ sudo dpkg-query -l | grep gnome-gmail-notifier *

You will get with gnome-gmail-notifier package name, version, architecture and description in a table

gnome-gmail-notifier package basic information: