The Problem

When a user clicks from shop.example.com to checkout.example-payments.com, GA4 treats this as two separate users by default. This causes:

If you spent money on a Google Ad that brought someone to your main site, and they then click to your checkout domain, GA4 reports the checkout traffic source as your main domain—not the paid ad. Your campaign ROI calculations become meaningless.

Cross-domain tracking solves this by passing user identifiers between domains.

When You Need Cross-Domain Tracking

You need it when:

You don't need it for:

Note: GA4 automatically handles subdomains. The tracking cookie is set on the parent domain and works across all subdomains without configuration.

How It Works

When cross-domain tracking is configured:

  1. User clicks a link from Domain A to Domain B
  2. GA4 appends a _gl parameter to the destination URL
  3. Domain B reads this parameter and connects the user to their existing session
  4. Both page views are attributed to the same user and session

Example URL with linker parameter:

https://checkout.example.com/cart?_gl=1*1abc2de*_ga*MTIzNDU2Nzg5*_ga_ABC123*MTYxMjM0NTY3OC4xLjEuMTYxMjM0NTY3OS4w

The _gl parameter contains encoded cookie values that identify the user.

Setup Method 1: GA4 Admin Interface (Recommended)

The simplest approach uses GA4's built-in configuration—no code changes required.

Prerequisites

Configuration Steps

  1. Go to Admin > Data Streams
  2. Select your web data stream
  3. Click Configure tag settings (at the bottom)
  4. Click Configure your domains
  5. Add all domains that should share user identity:
    • example.com
    • checkout.example-payments.com
    • app.example.io
  6. Click Save

Important: Include ALL domains in your user journey. Missing even one domain breaks the chain.

GA4 automatically:

Setup Method 2: Google Tag Manager

Use GTM when you need more control or when the GA4 admin method isn't sufficient.

Step 1: Configure Your Google Tag

Open your GA4 Configuration tag (or Google Tag) in GTM.

Under Configuration Settings, add:

Field Name Value
linker See below

The linker setting should be an object. In GTM, you can set this using a Custom JavaScript variable that returns:

function() {
  return {
    domains: ['example.com', 'checkout.example-payments.com', 'app.example.io'],
    accept_incoming: true
  };
}

Or use the Fields to Set option with these individual fields:

Field Name Value
allowLinker true

And add domains in GA4's admin interface.

Step 2: Verify on Destination Domain

The destination domain needs to accept the linker parameter. If you've added domains in GA4's admin interface, this is automatic.

If configuring purely through GTM, ensure:

Step 3: Publish and Test

Publish your GTM container changes and proceed to testing.

Testing Cross-Domain Tracking

Test 1: Check for _gl Parameter

  1. Navigate to your source domain
  2. Click a link that goes to your destination domain
  3. Check the URL in the browser address bar

Expected: URL contains ?_gl= or &_gl= parameter

If missing: Cross-domain linking isn't configured or the link type isn't supported (see Limitations).

Test 2: Verify Cookie Consistency

  1. On source domain, open DevTools > Application > Cookies
  2. Find the _ga cookie and note its value
  3. Click through to destination domain
  4. Check the _ga cookie value on destination domain

Expected: Same value on both domains

If different: The linker parameter isn't being read correctly

Test 3: Tag Assistant Verification

  1. Open Tag Assistant
  2. Connect to your source domain
  3. Navigate through your cross-domain flow
  4. Check that events appear in a single continuous session

Test 4: GA4 DebugView

  1. Enable debug mode
  2. Navigate between domains
  3. In GA4 DebugView, verify events from both domains appear under the same device

Test 5: Real-Time and Reports

After 24-48 hours, check:

Troubleshooting Common Issues

Issue 1: Self-Referrals Still Appearing

Symptoms: Your domain shows as a traffic source in acquisition reports

Causes:

Solution:

  1. Double-check domain list in GA4 Admin
  2. Verify same Measurement ID on all domains
  3. Clear browser cookies and retest

Issue 2: _gl Parameter Missing

Symptoms: Clicking cross-domain links doesn't add _gl to URL

Causes:

Solutions:

For JavaScript/button navigation, manually decorate the URL:

// Get the linker parameter
const linkerParam = ga.getAll()[0].get('linkerParam');

// Append to destination URL
window.location.href = 'https://checkout.example.com/cart?' + linkerParam;

Or use GTM's link decoration for non-standard elements.

Issue 3: _gl Parameter Stripped by Redirects

Symptoms: _gl appears momentarily then disappears

Causes:

Diagnosis:

  1. Install the Redirect Path Chrome extension
  2. Click your cross-domain link
  3. Watch for redirects that strip the _gl parameter

Solution: Work with your developers to preserve URL parameters through redirects. The redirect logic needs to forward all query parameters, not just known ones.

Issue 4: Payment Gateway Breaking Sessions

Symptoms: Users returning from Stripe/PayPal/etc. show as new sessions

Causes:

Solutions:

If you control the payment gateway domain (self-hosted):

If third-party gateway (Stripe Checkout, PayPal):

Path: Admin > Data Streams > Configure tag settings > List unwanted referrals

Add: stripe.com, paypal.com, etc.

Issue 5: Form Submissions Not Working

Symptoms: Form POST to another domain doesn't include _gl

Cause: Cross-domain linking only works for link clicks (<a> tags), not form submissions

Solution: Decorate the form action URL with the linker parameter:

// Before form submission
document.querySelector('form').addEventListener('submit', function (e) {
  const linkerParam = ga.getAll()[0].get('linkerParam');
  this.action = this.action + (this.action.includes('?') ? '&' : '?') + linkerParam;
});

Issue 6: GTM and GA4 Configuration Mismatch

Symptoms: Inconsistent behavior across domains

Cause: Domains listed in GA4 admin don't match GTM configuration

Solution: Pick one source of truth:

Handling Multiple Properties

Sometimes you need cross-domain tracking between sites using different GA4 properties (e.g., after an acquisition).

Challenge: Cross-domain tracking requires the same Measurement ID on all domains.

Solutions:

  1. Consolidate properties: Move to a single GA4 property for all domains (recommended)

  2. Dual tagging: Install both Measurement IDs on all domains, but only one handles cross-domain users. This creates duplicate data but maintains cross-domain continuity for one property.

  3. Accept the limitation: If properties must stay separate, cross-domain tracking won't work. Focus on referral exclusion to at least prevent self-referrals.

Edge Cases

Domains with Different Cookie Consent

If Domain A has consent granted but Domain B doesn't (user hasn't interacted with consent banner yet), the _gl parameter is passed but may not be used until consent is granted on Domain B.

Solution: Ensure your consent implementation on Domain B checks for existing linker parameters and honors them once consent is granted.

iFrames Across Domains

Cross-domain tracking doesn't automatically work for iFrames. The parent page and iframe are isolated contexts.

Solution: Post the linker parameter from parent to iframe via postMessage, then use it to initialize GA4 in the iframe.

AMP to Non-AMP

AMP pages have their own GA4 implementation. Cross-domain tracking from AMP to standard pages requires AMP-specific configuration.

Solution: Use AMP's linker configuration in your amp-analytics component.

Verification Checklist

Before going live, confirm:

Related Resources

Sources

#GA4 #Google Tag Manager #GTM #Cross-Domain Tracking #Attribution