Clicking link mailto: my.name@domain.com on my Google Site does not open gmail anymore. Has something changed? Thanks for information!
I don't know that which browser you are using. May be it's a browser specific problem.
Check with different browsers to see if it's same for all.
Also, try this link. It may help you -
http://googlesystem.blogspot.in/2012/02/open-mailto-links-using-gmail-in-google.html