What is _gl?

Who adds it

Google's tag (gtag.js, Tag Manager or Google Analytics) adds it to links between domains a site owner has set up for cross-domain measurement. It's added on the click, as the visitor moves from one domain to another.

What it means

The visitor came from another domain run by the same site owner, such as a separate shop or booking domain. It carries first-party cookie IDs so Analytics counts one user and one session across both domains.

What to do with it

Let it through: redirects that drop it break the cross-domain session. If those domains show up as referrers in Analytics, list them as unwanted referrals.

How Vireo shows it

Vireo doesn't store click IDs, in the free plugin or in Pro.

In the free plugin, Vireo drops the query string from page addresses, keeping only a few WordPress ones such as p and page_id, so a tagged URL counts as the same page as the untagged one. The visit is counted under the referrer it arrived with.

See Referrers and sources in the docs.

Sources

the cookies retain the same IDs as they are passed from one domain to another via a URL parameter ( _gl ) when the user navigates between domains through a link or a form.

Set up cross-domain measurement - Analytics Help

Cross-domain measurement with Google tags will add a linker parameter to URLs that point to the destination domain.

Measure activity across multiple domains - Google for Developers

Identify unwanted referrals - Analytics Help