It has been a long time since I wanted use Apples mail.app. But it still opened whenever I clicked on one of these mailto: links in a webpage. I googled and found that there is gmailto, which should do the trick, but is broken it seems. I could install it, gmail would open, but not understand the email address I wanted to send to.
Then I found Webmailer. This one works like a charm. It together with the gmail option to send email as andreas@andreaswacker.com make gmail pretty much complete. I am sure it breaks in about 7 minutes, when I rave about it like this.