1) Open Gmail in Chrome and click the Protocol Handler icon overlapping-diamonds in your browser's address bar. If you don't see this icon, skip ahead to step 3.

gmail-inbox

2) When prompted to "Allow Gmail to open all email links?", select "Use Gmail," and click "Done." Then you're done! Skip steps 3-5.

gmail-open-links

3) If you were unable to see the Protocol Handler icon overlapping-diamonds, click the Chrome menu icon chrome-menu-1 in the top right corner of your browser, and choose "Settings."

chrome-menu-dropdown-settings

4) Click the "Show Advanced Settings" link at the bottom of the screen. Then click the "Content Settings" button under the "Privacy" header.

privacy-settings

5) In the pop-up window, scroll down to the "Handlers" section and click the "Manage Handlers" button. Then select Gmail from the mailto dropdown, click "Done," and, well, you're done! 

manage-handlers

Note: If you're still having trouble (for example, your "Protocol handlers" window appears blank with no options), try installing Google's Mailto Extension for Chrome instead