The Gmail Notifier is an application that alerts you when you have new Gmail messages.When a new mail has arrived in your inbox a small window will pop up, showing author and subject of the newest mail. The underlined link will take you right to your inbox (using your preferred browser).
It also provides some additional features such as audio notification and support for LEDs on notebooks and multimedia keyboards.
Note:the Gmail Notifier isn’t compatible with the Google Toolbar.
Go to Applications->Add/Remove,search “gmail notifier”(without quotes) and mark to install.

After installed this,launch from Applications->Internet->Gmail Notifier.When you first launch this application,you’ll get this window for setting your account.

And you’ll see this icon on top right of screen.Double click it to check gmail,or right click for options.

the notification appears at the right and bottom edge: