That's what I thought. But maybe not. When I type mailto: into the Safari address window it goes back to Mail. There might not be a way to tell the System that mailto links are supposed to go to a web page.
This works for me - I have outlook set as default email client and web links / email links etc go directly to outlook not apple mail even when clicked within Safari browser.
I think it is less about gmail address and more about the email client selected in mail preferences - ie outlook, Thunderbird etc.