Cross-Domain Tracking

Cross-domain tracking solves a specific problem: a user navigates from yourstore.com to checkout.yourpaymentpartner.com and back, and you want GA4 to treat this as one continuous session rather than two separate users. Without cross-domain tracking, the checkout step shows as a direct-to-checkout visit with no attribution, and your funnel data is fundamentally broken.

Before configuring anything, confirm you actually need cross-domain tracking. Most “cross-domain” problems are actually subdomain problems, which are handled differently.

Do you actually need cross-domain tracking?

Subdomains (same parent domain): shop.example.com → checkout.example.com → account.example.com. These share cookies at the parent domain level (example.com). You do not need cross-domain tracking. GA4 cookies are set on the root domain and are accessible across all subdomains. Just make sure your GA4 configuration uses .example.com as the cookie domain.

Separate domains: yourstore.com → checkoutplatform.com. These have completely separate cookie namespaces. You do need cross-domain tracking.

Third-party checkout providers: Shopify Payments, Stripe Checkout, PayPal — these hosted payment pages are on different domains. Cross-domain tracking into a fully hosted third-party payment page is usually not possible (they’d need to include your GTM container). Instead, use server-side conversion tracking and exclude the payment domain from your referrals list.

How cross-domain tracking works

When a user clicks a link from Domain A to Domain B, GA4 appends a _gl parameter to the URL:

https://domainb.com/checkout?_gl=1*randomhash*...

The _gl parameter encodes the GA4 client_id and session information. When Domain B’s GA4 tag initializes, it reads _gl from the URL and uses the encoded client_id instead of creating a new one. The session is preserved.

This process is called “link decoration” and it happens automatically once you configure the domains.

Configuration via the Google Tag

In GTM, the cross-domain configuration lives in your Google Tag (GA4 Configuration Tag):

1. Open your Google Tag in GTM.

2. Under Configuration Settings, find Cross Domain. If you don’t see it, click “Show more settings.”

3. Add your domains. List all domains that are part of the same user journey. Format: example.com, checkoutsite.com, yourpartner.com. Do not include https://, paths, or trailing slashes.

4. Accept Incoming Linker: Ensure this is enabled (it usually is by default). This allows each domain to read the _gl parameter from incoming links.

5. Linker Decoration: Leave on Automatic. This decorates links to your configured domains automatically.

6. Publish the container (or update it on all relevant domains).

Both Domain A and Domain B must have this configuration with each other listed as cross-domain targets.

Verifying it works

After setup, navigate from Domain A to Domain B by clicking a link. Check the URL in your browser’s address bar on Domain B. You should see ?_gl=... appended.

If you don’t see _gl, the most common causes are:

1. The link navigates via JavaScript (window.location.href = '...') rather than an <a> element
2. The Google Tag isn’t loading on Domain A (check Preview mode)
3. The destination domain isn’t listed in your cross-domain configuration
4. A redirect is stripping the _gl parameter before the user reaches Domain B

In GA4 DebugView, you can verify by watching events on Domain B. If session_id and the first page_view on Domain B show the same session ID as Domain A, it’s working.

When _gl gets stripped by redirects

A redirect chain yourstore.com → redirect.example.com → checkout.com loses the _gl parameter if the redirect server doesn’t preserve query strings. This is a common failure point with:

• Marketing redirect tools (UTM shorteners)
• Affiliate link tracking systems
• Server-side 301/302 redirects that strip query parameters
• App deep link handling

The solution: ensure your redirect logic preserves all query parameters, or use server-side session bridging (pass the client_id in a cookie or server-side parameter rather than in the URL).

Excluding referrals in GA4

When cross-domain tracking is configured, the domains in your configuration should be excluded from GA4’s referral traffic reports (otherwise, returning from your payment provider looks like a referral visit, restarting the session and breaking attribution).

In GA4 → Admin → Data Streams → [your stream] → Configure Tag Settings → List unwanted referrals:

Add all domains involved in your cross-domain setup:

• checkoutplatform.com
• paymentprovider.com

Any session that starts with a referral from these domains will not start a new session.

Forms that cross domains

Standard link decoration only works for <a href> elements. If users submit a form that posts to a different domain, the _gl parameter is not automatically added.

For form-based cross-domain navigation:

// Add _gl parameter to form action URL before submission
document.querySelectorAll('form[data-cross-domain="true"]').forEach(function(form) {
  form.addEventListener('submit', function(e) {
    // GA4's linker API adds _gl to the action URL
    if (window.ga && window.ga.getAll) {
      var trackers = window.ga.getAll();
      if (trackers.length > 0) {
        var linker = new window.gaplugins.Linker(trackers[0]);
        form.action = linker.decorate(form.action);
      }
    }
  });
});

With GA4 (via gtag.js), use the built-in linker:

gtag('get', 'G-XXXXXXXXXX', 'linker', function(linker) {
  form.action = form.action + (form.action.indexOf('?') !== -1 ? '&' : '?') + linker;
});

Common mistakes

Configuring cross-domain for subdomains

shop.example.com and account.example.com share the same parent domain. GA4 cookies already work across these without cross-domain configuration. Adding them to the cross-domain list is harmless but unnecessary.

Only configuring one domain

Cross-domain tracking is bilateral. If you add checkoutpartner.com to yourstore.com’s configuration but don’t add yourstore.com to checkoutpartner.com’s configuration, the linker is only written on one side. Both domains need each other listed.

Forgetting the unwanted referrals list

Without the referral exclusion, returning from your checkout provider to your main site creates a new session attributed to the checkout provider as a referral. Your funnel shows users “coming from checkoutplatform.com” in the middle of their session.

Assuming it works without testing

Test cross-domain tracking with every browser, every redirect path, and — critically — with the payment provider completing a full checkout flow in staging. The _gl parameter is only as reliable as the weakest link in your redirect chain.

Related articles

User ID Tracking An alternative/complement to cross-domain: User-ID bridges identities across devices and sessions.

GA4 Configuration Tag The Google Tag where cross-domain settings live — full configuration reference.

Common Errors Cross-domain tracking issues appear here — referral problems, _gl stripping, session restarts.