So I did follow @robodome2's answer, but there may also be a situation where Chrome has set Gmail or another email app as a default client via use of a "handler".
This Chrome "handler" seems to override certain options, like selecting Apple Mail as the default client within Apple Mail app's options, as it is wholly within Chrome's settings.
I simply had to right-click and there was an option asking me how to "open email link as..." and then it took me to the "handler" page. I simply removed Gmail (and Outlook was on there?) and viola... Apple's Mail app is now the default.
I suspect if you go the the "handler" options within Chrome you can manually change/delete.
Handlers can be found:
Settings --> Privacy and Security --> Site Settings --> In the "Permissions" section you'll find "Handlers"
Hope this helps!