Set Attachment Image File URL as GUID

I have a form that uploads images and i want to update the GUID to be the attachment file url after WP generates the metadata.

I tried to use wp_update_post, but seems that is not updating the guid.

$attachments = get_posts($post_data);

if ($attachments) {
    // attach images to gallery
    foreach ($attachments as $attachment) {

        wp_update_post(array(
                'ID' = $attachment-ID,
                'post_status' = 'inherit',
                'post_parent' = $gallery_post_id,
                'guid' = wp_get_attachment_url($attachment-ID);
            )
        );
    }
}

Any suggestions ?