Can't 'Send through Gmail' from Cases.

We've enabled 'Send through Gmail' under the 'Send through External Email Services' option.

As a user in Service Console we can now go to a Contact, compose a new email, and it allows us to send through Gmail correctly.

However when we are in Cases, and we compose an email from the Case, or reply to an existing email, it is sending through Salesforce rather than Gmail. We can't see how to make these go through Gmail..