If you want to send them a confirmation email so they can confirm their membership in the list, then you need to remove the following code from your api call:
:double_optin => false
Mailchimp's API documentation states that the double optin option defaults to true. Gibbon's gem does not change this behavior.
Of course you can just change the value to true to be more explicit, but for future readers, it's important to know that the default setting is true so you are not caught off guard.