Help posting values to DB on submit using $wpdb->query

$wpdb has an insert method, so you can try the following:

$table="wp_email_subscribers";
$data = array(
            'first_name' => $firstname, 
            'last_name' => $lastname, 
            'email'=> $email , 
            'gdpr_consent'=>$gdprconsent 
       );
$format = array('%s','%s', '%s', '%s');
$wpdb->insert($table,$data,$format);
var_dump($wpdb->insert_id);