If you mean you want a "mailto" link to open your browser to the Gmail website and create a new message to the linked recipient then this page, methods untested by me, might be of some help:
If you mean you want a "mailto" link to open your browser to the Gmail website and create a new message to the linked recipient then this page, methods untested by me, might be of some help:
Because…you can't get to the Mail preferences without setting up an account! One of the stupid things that hasn't been changed in macOS. They have an app called "System Preferences" but they don't even put your default browser or email preference selector in it.
To get to that setting you're going to have to add a Mail account. Do you have your iCloud account set up on there? Enabling "Mail" in for iCloud in System Preferences>Internet Accounts will do the trick. Once you've got the Gmail thing set up (and I still don't know if it works) I think you can disable the iCloud Mail and the default email app setting should stick…you'll find out for us.