You no longer need to rely on the extensions to get desktop notifications for Gmail, as Gmail now it self offers HTML5 desktop notifications which display pop-ups whenever a chat message or new email arrives.

Gmail's desktop notifications

Currently Google Chrome is the only browser that supports desktop notifications for Gmail, to enable them click Settings and scroll down to “Desktop Notifications”,  in this section you can customize notifications you wish to receive like Chat notifications, new mail or important mail notifications if you are using Priority inbox.

enable desktop notifications for Gmail