How to Set Up Cross-Domain Tracking with Google Tag Manager

If you run a small business with multiple websites or checkout platform you might be missing key insights in your Google Analytics 4 (GA4) reports.

Contents

What Is Cross-Domain Tracking And Do You Even Need It?Let’s break it down with an example: Quick Checklist: Do You Need Cross-Domain Tracking?What You'll Need Before You Start Step-by-Step: How to Set Up Cross-Domain Tracking in GTM Step 1: Add Domains to Your GA4 Configuration Tag in GTM Step 2: Add Domains in GA4 Admin Settings Step 3: Enable Auto Link Domains How to Test If It's Working Option 1: Use GTM Preview Mode Option 2: Use Tag Assistant in Chrome Troubleshooting: Common Issues and Fixes ❌ Sessions Still Splitting?❌ Referral Traffic Showing Up From Your Own Site?❌ Page Paths Look Broken?Real-World Example: Online Shop & External Checkout

Ever notice your traffic numbers don’t add up? Or that visitors suddenly become “new users” halfway through their journey?

That’s where cross-domain tracking comes in.

And don’t worry, it sounds more technical than it really is. I’ll guide you through the process using Google Tag Manager (GTM), real-life examples, and step-by-step instructions you can follow today.

What Is Cross-Domain Tracking And Do You Even Need It?

Cross-domain tracking connects user sessions across multiple domains so Google Analytics sees them as one journey, not separate visits.

Let’s break it down with an example:

You own a service business and your main website is:

www.mybookstore.com

You take bookings through a third-party system like:

mybookstore.bookingplatform.com

Without cross-domain tracking, GA4 sees this as two separate users and two sessions even though it’s the same person!

That means:

Your source/medium attribution gets lost
Your funnels break
Your conversion data is incomplete

Quick Checklist: Do You Need Cross-Domain Tracking?

✅ You send users between two or more different domains

✅ You use third-party platforms for checkout or forms

✅ You want clean attribution and complete user journeys in GA4

If you said yes to any of these—you’re in the right place!

What You'll Need Before You Start

Before we dive in, here’s what you’ll need:

Access to Google Tag Manager (GTM)
Access to your GA4 account
A list of all domains you want to track
(Optional) Chrome + Tag Assistant Extension for testing

Step-by-Step: How to Set Up Cross-Domain Tracking in GTM

Step 1: Add Domains to Your GA4 Configuration Tag in GTM

Open Google Tag Manager
Go to Tags → GA4 Configuration
Under Fields to Set, add:
- Field Name: allowLinker → Value: true
- Field Name: linker → Value: { 'domains': ['domain1.com', 'domain2.com'] }
Replace domain1.com and domain2.com with your actual domains.

Step 2: Add Domains in GA4 Admin Settings

Go to GA4 → Admin → Data Streams
Click your data stream
Scroll to Tagging Settings → Click Configure your domains
Add all domains you want to track, like:

www.mybookstore.com

booking.mybookstore.com

Step 3: Enable Auto Link Domains

If you didn’t do it manually in GTM, GA4 can try to auto-decorate links:

In GA4 → Admin → Data Stream
Under Tagging Settings, enable Cross-domain auto linking

Manual setup is more reliable, especially if you have embedded forms or iframes.

How to Test If It's Working

Now that you’ve set it up, let’s test it to make sure it works.

Option 1: Use GTM Preview Mode

Click Preview in GTM
Enter your main website URL
Navigate through to the second domain
Make sure the GA4 configuration tag fires properly

Option 2: Use Tag Assistant in Chrome

Install the Google Tag Assistant
Start recording
Click from one domain to the next
Check that the client ID stays the same in both GA4 hits

Troubleshooting: Common Issues and Fixes

❌ Sessions Still Splitting?

Double-check that allowLinker is set to true in GTM
Verify that you added all domains in both GTM and GA4

❌ Referral Traffic Showing Up From Your Own Site?

Go to GA4 → Admin → Data Settings → Traffic definitions
Add your domains under List unwanted referrals

❌ Page Paths Look Broken?

Consider setting a custom dimension to track full URLs

Real-World Example: Online Shop & External Checkout

Let’s say you run an ecommerce store:

Main domain: www.handmadebyjen.com
Checkout: checkout.shopify.com

Without cross-domain tracking, you won’t know which Instagram ad drove the sale—because GA4 loses the referral data as soon as someone moves to checkout.shopify.com.

Once cross-domain tracking is properly set up, you can follow that visitor from Instagram → product page → checkout → purchase in one seamless report.

Congrats! You just leveled up your GA4 tracking with cross-domain setup using Google Tag Manager.

Now you’ll be able to:

Accurately track full user journeys
Improve your marketing attribution
Make better data-driven decisions