NotifyOSD is useful for quick notifications and is used by almost all applications available in Ubuntu Software Centre except the web-browsers. You do have browser based popup notifications but they are sometimes obtrusive and apprear from the wrong places. If you are using Google Chrome and want to use NotifyOSD to receive notifications for your chats and emails, you can do so easily.
Google Chrome supports apps and extensions and one such extension, LinuxNativeNotification, can be used to channel notifications for email and chats to NotifyOSD.
|Google Chat/Hangout Notificaion|
To download the extension, head over to the Chrome Apps store and search for Linux Native Notification or just click on this link. Download and install the extension. Also make sure to turn off other notifications for Gmail from the Gmail settings page.
Note: This notification works for only one gmail account per session. If you have multiple gmail sessions, it will notify only from the first opened gmail session.