OS X - Safari
To specify your default web browser and email app, see
https://support.apple.com/en-us/HT201607
To set Gmail as Your Default Email Client, see
http://blog.hubspot.com/marketing/set-gmail-as-browser-default-email-client-ht,
which downloads
https://sites.google.com/site/mailtoextension/mailto.safariextz.