$ sudo apt-get update
$ sudo apt-get install gnome-shell-pomodoro 

gnome-shell-pomodoro is installed in your system.

Make ensure the gnome-shell-pomodoro package were installed using the commands given below,

$ sudo dpkg-query -l | grep gnome-shell-pomodoro *

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

gnome-shell-pomodoro package basic information: