One of the best features of Gmail is the built-in keyboard shortcuts. They allow you to do everything from composing a message to quickly archiving messages.
In order to use keyboard shortcuts, they must be enabled in Gmail settings. To do this, first hit the gear icon at the top right of the Gmail screen, and then go to the Settings.
Scroll midway down the Settings, and make sure that Keyboard Shortcuts are turned ON. Be sure and scroll to the bottom and SAVE your settings.
Now, the keyboard shortcuts will be enabled for you. Try them out… hit the C on your keyboard to begin composing a new message. When you are done, hit Control-Enter to send your message. On a Mac, you’ll use the Command key instead of Control.
To see a full list of keyboard shortcuts, you can hit the ? (question mark) at any time. This will bring up a keyboard overlay as shown below.
This Google help link also has more info on the keyboard shortcuts: https://support.google.com/mail/answer/6594?hl=en