By default, when you click on an email link to send or compose a mail, normally your default desktop mail client Outlook, Thunderbird, or any other program will open up. But if you prefer Gmail instead of desktop mail client, now it is easy in Chrome – thanks to the magic of HTML5 things are getting simpler.
Now onwards clicking on email links in any application on your computer will open a fresh Gmail compose window. (Via)
You can also set other browsers to open in Gmail. Read it here more about it.