The unsubscribe link is automatically added to all emails sent from the automated emails ... like gallery expiring emails sent with the automated email option. There is no option to add them into the default emails.
Yes, if you have Automated Emails enabled in Settings -> Automated Emails, when you send the emails to the pre-registered people from the People tab the unsubscribe link is automatically added at the bottom of the email.
When you send people an email from the People tab in a gallery, it uses the automated email function if it is enabled to send out the emails so they are not sent all at once. AND it adds the Unsubscribe link to those emails when they are sent.
I've taken another look and I now see the unsubscribe links are being added to the gallery invite emails from the People tab. I thought I had it adding when they cron job sent the email but that's not the case. I will have to add it for the next update.