Change content of email verification email based on gender provided by user
You've got two options:


Create yourself a custom token and use that.
Use the hook_mail_alter function


https://api.drupal.org/api/drupal/modules%21system%21system.api.php/function/hook_mail_alter/7

The token approach is the more elegant approach, but needs a bit of head-wrapping to understand the concepts. The hook_mail_alter might be an easier quick hack.