Apple/iPhone requires users to use Mac Mail as their default email app. As a result, only Mac Mail will open automatically when you click "email" to claim a lead in Connect Now.
Even if you primarily use gmail and the gmail app for email, you can set up Mac Mail to use Gmail so that your lead response is quick and seamless.
Follow these directions to add your gmail, gsuite or other work email address to Mac Mail.
To make sure that you automatically send from a particular account (your work account, rather than your personal, for example) follow these directions.