Solved with the attachment page also: don't know if it's the correct one:
functions.php
foreach ( $attachments as $id => $attachment ) {
$full_image = wp_get_attachment_url($id);
$thumbnail = wp_get_attachment_thumb_url($id);
$title = trim($attachment->post_excerpt) ? wptexturize($attachment->post_excerpt) : $attachment->post_title;
$attachment_page = get_attachment_link($id);
and then
if(gallery_option('galleryopt_lightbox_type') == "none") {
$output .= '<li><a href="'. $attachment_page .'"><img class="img" src="'. $thumbnail .'"></a></li>' . "\n";
It seem to work ok for now
