When a user configures a gravatar, the event defined for modifying the avatar image code is not hit.  So, it is impossible to modify the gravatar as other types of avatars.  Eventhough there is an event in the gravatar driver, this is not enough, as it does not have enough information available (and cannot be added, as it is not available in the driver).

Solutions: either avoid returning too early from the function get_avatar(), or fix the gravatar driver so that it does not use the "custom_url" method, as it is not needed at all.