function notification_emails_from_tag( $notification, $form, $entry ) {

    // only change notification to address for this specific notification
    if ( $notification['name'] == 'User Notification' ) {

        $tag = 'Tag Name';

        $args = array(
            'meta_query' => array(
                array(
                    'key'     => wp_fusion()->crm->slug . '_tags',
                    'value'   => '"' . $tag . '"',
                    'compare' => 'LIKE',
                ),
            ),
        );

        $users = get_users( $args );

        $emails = array();

        if ( ! empty( $users ) ) {

            foreach ( $users as $user ) {

                $emails[] = $user->user_email;

            }
        }

        // replace the to address with a comma separated list of emails
        $notification['to'] = GFCommon::implode_non_blank( ',', $emails );

    }

    return $notification;
}

add_filter( 'gform_notification_1', 'notification_emails_from_tag', 10, 3 );