A custom gallery link typically looks like this:

/gallery/galery_slug/index.html

When rotating galleries on your site, links include rotation parameters:

/gallery/galery_slug/index.html?1x2x1234

The parameter `1x2x1234` changes periodically.

Here's the issue: Google treats these URLs as different pages:

/gallery/galery_slug/index.html?1x2x1234 and /gallery/galery_slug/index.html?2x2x1234

As a result, Google periodically indexes both links, then later recognizes them as duplicates and removes one (or both).

To prevent this, add a canonical tag to your pages:

<link
  rel="canonical"
  href="http://yourdomaincom/gallery/<!--GALLERY_SLUG-->/index.html"
/>

There's another consideration: a visitor arriving directly at a URL such as /gallery/galery_desc_bla-bla/123455/index.html?2x2x1234 may be redirected according to your rules, including trade and traffic-sale rules. A rotation parameter in the URL triggers outgoing traffic processing. Without it, the script displays the gallery.

The safe URL is `/gallery/galery_desc_bla-bla/123455/index.html` — without rotation parameters. This URL always shows the gallery content. Search engine users accessing this URL directly will see the gallery, regardless of your SE traffic or sales settings.