If you prefer accessing Gmail from a browser rather than OS X's Mail app or another desktop app, then you miss out on the desktop notifications that alert you to the arrival of new email messages -- or so I thought. With the Aura app, you can get the best of both worlds: desktop notifications and Gmail's Web client. Aura is free in the Mac App Store for one Gmail account; a $4.99 in-app purchase lets you add multiple accounts.
After installing Aura and connecting it to your Gmail account, you'll see a new rocket icon in your menu bar that shows you the number of unread messages in your inbox. From here, you can access preferences where you can set Aura to open at login, choose to have Aura's notifications play a sound, and have Aura notify you only when important emails arrive (if you use Gmail's Priority Inbox feature).
With Aura running, you'll get OS X desktop notifications as messages arrive in your Gmail inbox. Clicking on a notification opens a new tab in your default browser with that message. Aura also gets its own section on your Mac's Notification Center.