How do I prevent a checkout domain from replacing acquisition data in GA4?

Definition: Cross-domain measurement connects activity across separate website domains as one user journey. As of October 2026, Google's documented approach for GA4 is to configure the storefront and checkout domains together, carrying user and session identifiers through the _gl URL parameter.

Key facts

• Google's cross-domain setup guide requires the same Google tag ID from the same web data stream on each participating domain.
• Google's unwanted-referrals documentation identifies third-party payment processors as a use case for referral exclusion.
• Google's session documentation says the session_start event carries the information that determines session attribution, including the referrer.

How are storefront and checkout domains connected?

Google's setup instructions place the configuration in Admin, under Data collection and modification, then Data streams. The web stream's Configure tag settings panel contains Configure your domains.

Each participating domain is added with a match type and domain condition, then saved. Conditions use OR logic. All participating pages need the same measurement ID from the same web stream, including checkout pages where the tag can be installed.

How does the linker handle links and forms?

The linker is the mechanism that transfers measurement cookie values between domains. Google's developer guide describes a source tag adding _gl to the destination URL and a destination tag reading it. With consent, this carries the identifiers needed to measure the visit as one user and session.

Google recommends the Analytics Admin configuration for GA4. For a custom gtag.js implementation, its examples support the following configuration, with placeholder domains replaced by the participating domains:

gtag('set', 'linker', {
  'domains': ['example.com', 'example-b.com'],
  'decorate_forms': true
});

The set command precedes gtag('js', new Date()) and the relevant config command. A populated domains array enables acceptance of incoming linker parameters by default. The decorate_forms setting enables decoration of forms submitted across domains; its default is false.

A linker object set in Tag Manager overrides the domains configured in Analytics Admin; its domains array must include the intended domains.

How is the checkout handoff checked?

Google's verification procedure tests an actual link click or form submission and checks the destination page loads with _gl. Redirects must preserve that parameter. Chrome Developer Tools network requests can expose its removal during a redirect. JavaScript navigation and scripts stopping click-event propagation can also interrupt decoration.

Google's linker guide gives this example:

https://www.example.com/?_gl=1~abcde5~

Linker values expire after two minutes and are added at click time. Testing the actual checkout action therefore matters more than inspecting an unclicked link's original HTML.

What if payment happens on a provider's domain?

Google's unwanted-referrals guide covers customers returning from a payment processor. In the web stream's Configure tag settings panel, Show all exposes List unwanted referrals. A condition matching the provider's domain excludes that referrer as a traffic source by applying ignore_referrer=true to matching events.

Cross-domain linking transfers identifiers; referral exclusion suppresses a source. For domains connected through cross-domain measurement, Analytics automatically suppresses the referral when the current page contains _gl.

Related terms

• GA4 traffic acquisition
• UTM parameters
• What happens if an AI assistant domain is added to GA4's unwanted referrals?

FAQ

Does a checkout subdomain also need attention?

Yes. Google's cross-domain documentation recommends checking that your subdomains share the same cookie domain; differing cookie domains can produce self-referrals.

Why can an excluded provider still appear in reports?

Google's referral-exclusion example shows a referral acquired before exclusion still receiving credit for a later direct return under last-non-direct-click attribution. Adding your provider to the list can therefore leave that earlier source visible.

Sources

1. Google Analytics Help, "Set up cross-domain measurement", undated
2. Google for Developers, "Measure activity across multiple domains", 13 October 2026
3. Google Analytics Help, "Identify unwanted referrals", undated
4. Google Analytics Help, "About Analytics sessions", undated

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