Try this:
(generally if you do the Firefox change or the Chrome change it will feed over into Safari, if for some reason Safari does not start using gmail after doing the Firefox or Chrome change then add the extension noted for Safari below).
For Firefox:
Firefox - Preferences - Applications - mailto: and choose use Gmail. Close and restart your browser.
For Chrome:
Log into your gmail in Chrome - click on the protocol handler (looks like two diamonds in the url) choose Allow to open mail - done.
For Safari:
Safari - Preferences - Extensions - Get Extensions and search for Mailto - install - choose Gmail as the email to send from and ok.
Close and restart the browser, find an email address to click on and it should open it in your gmail account.