Cross-domain tracking with Google Tag Manager

#01The problem in one story

User lands on www.brand.com from a Google Ad, browses, then checks out on pay.partner-checkout.com. Without linking, the purchase may look like a new direct or referral session, breaking attribution and confusing Smart Bidding.

Google’s cross-domain docs explain that tags decorate outbound URLs with a _gl parameter and the destination tag reads it to restore first-party cookies.

#02Configure domains on the Google tag

Per Google Tag Manager / Google tag settings Help:

1. Open Google tag settings → Configure your domains.
2. Add conditions for each domain in the journey (brand.com, partner-checkout.com).
3. Save.
4. Ensure both domains load compatible Google tags / GTM containers that accept linker params.

Accept recommendations when Google detects related domains — but still verify manually.

#03Conversion Linker auto link

In the Conversion Linker tag, enable linking across domains and list Auto Link Domains. That appends linker parameters to matching outbound links. Optional: decorate forms if POSTs navigate across domains.

#04Referral exclusions vs cross-domain

Different tools:

• Cross-domain: stitch identity across domains you measure as one journey.
• Unwanted referrals / referral exclusions: prevent gateways (PayPal, login SSO) from appearing as referrers when they should not restart attribution.

You often need both. See also GA4 referral settings discussions in the GA4 track.

#05QA checklist

1. From domain A, click to domain B.
2. Confirm destination URL contains _gl=.
3. Confirm page loads (rare server issues with long query strings — Google documents troubleshooting).
4. In GA4 DebugView, confirm the same client seems continuous (session debugging takes care).
5. Test primary buttons and key CTAs, not only a footer link.

Linker parameters expire quickly (Google notes a short expiry window measured in minutes), so they are added at click time.

#06Worked example: Ledgerline

Ledgerline is a fictional accounts-payable software company. Details are illustrative.

Ledgerline used a billing vendor on billing.ledgerline-pay.com. Paid conversions looked like referrals from the billing host. They added both domains to Google tag domain config, enabled Linker auto-link, and added the billing host to unwanted referrals as appropriate. Assisted conversion paths started showing the original Google Ads click again.

Walkthrough

1. List every hostname in the signup → pay → success path.
2. Add them to Configure your domains.
3. Update Conversion Linker auto-link list.
4. Publish to staging; click through with Tag Assistant recording.
5. Confirm _gl and conversion attribution on a test click.

#07Third-party checkouts you do not control

If the destination cannot install your Google tag, true cross-domain stitching may be impossible. Options then: payment provider integrations, gateway referral exclusion plus careful campaign tracking, or server-side matching later. Be honest in reporting about the limitation.

#08Form POSTs across domains

If a form POSTs to another domain, enable decorate forms on the Linker where applicable and test. GET navigations are easier to verify because you can see _gl in the address bar.

#09Debug recipe

1. Clear cookies.
2. Land on domain A with a fresh gclid.
3. Navigate to domain B via the primary CTA.
4. Confirm _gl present.
5. Complete conversion on B.
6. Confirm Ads/GA4 attribution still knows the click.

#10Linker configuration details

In the Google tag / Conversion Linker settings, list domains that should receive linker parameters. Include bare domains and www variants intentionally. Forgetting www vs apex is a classic break: users bounce between both and stitching fails half the time.

#11When _gl is stripped

Security tools, email scanners, and some redirectors strip query parameters. If marketing automation wraps links, test the full redirect chain. Intermediate domains that are not linker-aware will drop _gl.

#12Referral exclusion vs linking

Excluding a payment domain as a referral in GA4 prevents the payment host from showing as the source — useful, but it is not the same as preserving the original campaign click across domains. You often need both: linker for user stitching and referral exclusion for clean source reports (GA4 referrals lesson patterns still apply to channel design).

#13Subdomains

shop.example.com and www.example.com usually share cookies if the cookie domain is set to .example.com. Confirm Google tag cookie configuration. Subdomains are easier than separate root domains — do not over-engineer Linker if a cookie-domain fix suffices.

#14iFrames and embedded checkouts

If checkout is an iframe from another domain, Linker decoration on top-level clicks may never touch the iframe navigation. Options: vendor-native GA4 integration inside the iframe app, postMessage bridges designed with engineering, or accepting broken stitching and using server-side order matching. Be explicit in dashboards when iframe checkout means campaign→purchase is probabilistic.

#15Login walls across domains

SSO flows that bounce through auth.example.com need that host in the linker domain list too. Otherwise the auth hop drops _gl before the app domain loads. Map the full redirect graph once with a network waterfall and a test Google click.

#16Step-by-step linker QA script

1. Note domains in scope (example: www.ledgerline.example, app.ledgerline.example, checkout.ledgerline.example).
2. Confirm Google tag / Linker domain list includes each host you navigate across.
3. Open an incognito window; disable extensions.
4. Click a test ad or manually append a dummy gclid=testdebug001 on domain A (understand Ads may ignore fake gclids for reporting — you are testing decoration behaviour).
5. Navigate via the real CTA to domain B.
6. Confirm the URL on B contains _gl= (or that first-party cookies continued on subdomain without needing _gl).
7. Complete a test conversion on B.
8. In GA4 DebugView / Ads diagnostics, confirm the session is not a brand-new (direct) dead-end if you expected campaign continuity.
9. Record pass/fail in the inventory sheet with date.

#17WWW and HTTPS redirects

If http://example.com → https://www.example.com redirects strip query params incorrectly, _gl dies before GTM runs. Test the redirect chain with query strings. Fix redirects at the edge (CDN/load balancer) — GTM cannot recover parameters that never arrived.

#18Linker and consent together

Conversion Linker and Google tag linking still respect consent. If ad_storage is denied, advertising identifiers may not persist as they would under grant — which is intended. When debugging cross-domain Ads attribution, always re-test on a grant path. A deny-path "failure" is often compliance working correctly, not a broken domain list. Document both outcomes in QA notes so media buyers do not "fix" consent to chase attribution.

#19App deep links and store redirects

If a CTA sends users to an app store or opens an app via deep link, browser GTM cross-domain linking will not stitch that journey the way web-to-web _gl does. Measure app conversions with the appropriate SDK / Firebase / AppsFlyer-style stack and accept a break in the web session. Trying to force Linker onto store URLs wastes time — document the handoff as a known measurement boundary for media buyers.

#20Try it yourself

Map your funnel hostnames. If more than one registrable domain appears, schedule cross-domain QA this week.

#21Quick quiz

1. What query parameter should appear on linked destination URLs?

Answer: _gl.

2. Are `www` and bare domain different registrable domains?

Answer: Usually the same registrable domain; still list what you actually use and test redirects.

3. Cross-domain vs referral exclusion?

Answer: Cross-domain stitches identity; referral exclusion stops unwanted referrer resets.

#22What should you check before the next lesson?

• Domain list complete.
• _gl verified on a real click path.

Previous: Ecommerce data layer. Next: Debug with Preview and Tag Assistant. Or return to the GTM for Marketers track.

#23Common questions

When needed? Multi-domain journeys under your measurement.

Subdomains? Usually not "cross-domain" in the registrable sense.

`_gl`? Linker parameter for cookie restoration.