Dynamically add to Gravity Forms notification email address list (include emails stored in a custom post meta field).

functions.php

// change # to the id of your form to target a specific form
add_filter( 'gform_notification_#', 'route_notification', 1, 2 );
function route_notification($notification, $form , $entry) {
    global $post;
    // the $post here is the post where the Form is displayed
    $email_to = get_post_meta($post->ID, 'email', true);
    if ($email_to){
        $notification['to'] .= ',' . $email_to;
    }
    return $notification;
}