Developer here. I just shipped this app that makes email links open into your Gmail, Yahoo, etc. instead of Apple Mail.
I made this because the old "mailto" extension for Safari stopped working with Safari 12 / Mojave. (Apple blocked legacy extensions.). Works system-wide for Safari, Chrome, or Firefox. Though, if you're a Chrome or Firefox user and use Gmail, there are other ways to accomplish this without an extra app.
It's a really simple app, but I spent some extra time packaging it up nicely and hope it's useful to some of you!
Mac App Store —
https://itunes.apple.com/us/app/open-in-webmail/id1451552749?mt=12
Web Site —
https://fpotter.org/open-in-webmail/