Gmail will display an icon of unread messages on the tab of your browser
If you still have a tab Gmail open your browser you may want to enable the "Icon unread. You will find this new section in Labs Gmail.
This service will display on the tab the number of unread messages, just below the icon for Gmail. Note that if you use the Inbox priority, only the messages in this box will be treated as unread messages. Only works with Chrome and Firefox browsers.