With the latest version of Chrome, when I click on a mailto: link, GMAIL no longer opens by default.

Here’s how to fix it:

1) Open GMAIL in Chrome
2) On the same page, right-click and click on "Inspect"
3) Go to the bottom in "Console"

4) Copy this function and paste it there under the "Console"
    

navigator.registerProtocolHandler("mailto",
                                  "https://mail.google.com/mail/?extsrc=mailto&url=%s",
                                  "Gmail");

5) Press Enter
6) A confirmation from Chrome will ask you to allow the mailto association with GMAIL again, click on Allow

And there you go