Source: http://lifehacker.com/5881208/add-hyperlinks-to-gmail-messages-with-a-keyboard-shortcut
Most word processors use the keyboard shortcut Ctrl+K (or Cmd+K on Mac) to insert a hyperlink into a text, but even though the shortcut exists in Google Docs, it never worked in Gmail. Now it does.
It's the exact same keyboard shortcut you use in Word, Outlook, Apple Mail, Google Docs, and nearly every other word processing and email platform. To make the best use of it, select the text you want to turn into a link, hit Ctrl+K, and then paste the URL you want to link to. Now you have a handy hyperlink in your email without scaring off your recipient with a garbled URL. If you're looking for a breakdown of a few more handy Gmail shortcuts and tricks, check out our guide to mastering Gmail.
Gmail's Shortcut for Inserting Hyperlinks | Google Operating System