Multiple Google Tag Manager (GTM) Installation

TL;DR

Using multiple GTM containers has been officially supported since 2015. However, all containers must share the same dataLayer — using different dataLayer names causes trigger issues and data loss. Modern GTM implementations require Consent Mode v2 (GDPR/CCPA), Server-side tagging, and GA4 event schema as essential components.

Topic	2019 Approach	2025+ Current State
dataLayer	Different name per GTM	Single dataLayer required
Instantiation	dataLayer = [{...}]	window.dataLayer = window.dataLayer || []
Analytics	Universal Analytics	GA4 (UA deprecated)
Privacy	Optional	Consent Mode v2 required
Tagging	Client-side only	Server-side preferred

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Why Multiple GTM Is Needed?

In most cases, a single GTM container is the best solution. However, multiple containers may be unavoidable in these scenarios:

• Different teams/agencies: Ad agency wants to manage their own tags
• Access control: Only certain people should have access to specific tags
• Multi-tenant structures: Different business units need independent tracking on the same domain
• Vendor requirements: Third-party software mandates their own GTM

---

dataLayer: Legacy vs Current Approach

Legacy Approach (2019)

2019 recommendation: Different dataLayer names could be used for each GTM container. For example, first container uses dataLayer, second container uses newLayer.

<!-- 2019: Different dataLayer names (NO LONGER RECOMMENDED) -->
<script>(...,'dataLayer','GTM-XXXX');</script>
<script>(...,'newLayer','GTM-YYYY');</script>

Current Approach (2025+)

Google’s current documentation clearly states:

“Using more than one data layer can cause some triggers to stop working and could have other implications.” — Google Tag Manager Developer Guide

All containers must share the same dataLayer:

<!-- Correct: Single dataLayer, multiple containers -->
<script>
window.dataLayer = window.dataLayer || [];
</script>

<!-- GTM Container 1 -->
<script>(function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':
new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],
j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src=
'https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);
})(window,document,'script','dataLayer','GTM-XXXX');</script>

<!-- GTM Container 2 -->
<script>(function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':
new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],
j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src=
'https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);
})(window,document,'script','dataLayer','GTM-YYYY');</script>

dataLayer Instantiation

Legacy method (problematic):

dataLayer = [{'pageTitle': 'Home'}];

This usage completely overwrites the existing dataLayer.

Correct method:

// Safe instantiation - preserves existing data
var dataLayer = window.dataLayer = window.dataLayer || [];

// Always add data using push()
dataLayer.push({
  'pageTitle': 'Home',
  'pageType': 'home'
});

---

Key Considerations for Multiple Containers

1. Shared dataLayer = Shared Events

Since all containers share the same dataLayer, events pushed by one container are visible to other containers. This means:

Advantage: Ensures data consistency Risk: Unintended tag triggers may occur

2. Event Naming Convention

// ❌ Wrong: Generic event name
dataLayer.push({ 'event': 'click' });

// ✅ Correct: Specific and namespaced
dataLayer.push({ 'event': 'brand_cta_click' });
dataLayer.push({ 'event': 'agency_form_submit' });

3. PII (Personal Data) Risk

All data pushed to the shared dataLayer is accessible by all containers. If one container pushes PII (name, email, phone), other containers can also access this data.

// ⚠️ Warning: This data is visible to ALL containers
dataLayer.push({
  'event': 'form_submit',
  'userEmail': 'user@example.com', // PII!
  'userName': 'John Doe' // PII!
});

---

Consent Mode v2 Integration

Consent Mode v2 is now mandatory for GDPR, CCPA, and other privacy regulations.

Basic Setup

// BEFORE the GTM snippet
window.dataLayer = window.dataLayer || [];
function gtag(){dataLayer.push(arguments);}

// Default consent state (before user permission)
gtag('consent', 'default', {
  'ad_storage': 'denied',
  'ad_user_data': 'denied',
  'ad_personalization': 'denied',
  'analytics_storage': 'denied'
});

// Update after user grants permission
gtag('consent', 'update', {
  'ad_storage': 'granted',
  'ad_user_data': 'granted',
  'ad_personalization': 'granted',
  'analytics_storage': 'granted'
});

Regional Consent Settings

// Default denied for EU users
gtag('consent', 'default', {
  'ad_storage': 'denied',
  'analytics_storage': 'denied',
  'region': ['AT', 'BE', 'BG', 'HR', 'CY', 'CZ', 'DK', 'EE',
             'FI', 'FR', 'DE', 'GR', 'HU', 'IE', 'IT', 'LV',
             'LT', 'LU', 'MT', 'NL', 'PL', 'PT', 'RO', 'SK',
             'SI', 'ES', 'SE']
});

// Granted for other regions
gtag('consent', 'default', {
  'ad_storage': 'granted',
  'analytics_storage': 'granted'
});

---

GA4 Event Schema (Replacing Enhanced Ecommerce)

Legacy approach (Universal Analytics Enhanced Ecommerce):

dataLayer.push({
  'event': 'addToCart',
  'ecommerce': {
    'currencyCode': 'USD',
    'add': {
      'products': [{ 'id': '123', 'name': 'Product', 'price': 99.90 }]
    }
  }
});

GA4 Ecommerce Event Schema:

dataLayer.push({ ecommerce: null }); // Clear previous ecommerce data

dataLayer.push({
  'event': 'add_to_cart',
  'ecommerce': {
    'currency': 'USD',
    'value': 99.90,
    'items': [{
      'item_id': '123',
      'item_name': 'Product Name',
      'item_brand': 'Brand',
      'item_category': 'Category',
      'price': 99.90,
      'quantity': 1
    }]
  }
});

GA4 Standard Events

Event	Description
view_item	Product detail page view
add_to_cart	Add to cart
remove_from_cart	Remove from cart
begin_checkout	Start checkout
add_payment_info	Payment info entry
add_shipping_info	Shipping info entry
purchase	Purchase completion

---

Server-Side GTM

Server-side GTM has become the standard for enterprise projects in 2025.

Advantages

• Performance: Reduces client-side script load
• Privacy: Data processing in first-party context
• Ad-blocker bypass: Server-side requests aren’t blocked
• Data control: PII masking and filtering

Basic Architecture

[Browser] → [Client-side GTM] → [Server-side GTM Container] → [GA4, Ads, Meta...]
                                        ↓
                                [First-party domain]

---

Platform Integrations: Shopify

Shopify runs GTM as a Custom Pixel in a sandbox environment. This provides security but introduces some limitations.

Shopify Sandbox Limitations

• Limited direct DOM access
• No document.cookie access
• Limited localStorage / sessionStorage
• Third-party scripts run inside the sandbox

GTM Custom Pixel Setup

// dataLayer and gtag definition
window.dataLayer = window.dataLayer || [];
function gtag(){dataLayer.push(arguments);}

// GTM snippet (without HTML tags)
(function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':
new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],
j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src=
'https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);
})(window,document,'script','dataLayer', 'GTM-XXXXXXX');

// Consent Mode v2
gtag('consent', 'update', {
  'ad_storage': 'granted',
  'analytics_storage': 'granted',
  'ad_user_data': 'granted',
  'ad_personalization': 'granted',
});

Forwarding Shopify Events to GTM

Use Shopify’s analytics.subscribe() API to listen for standard events and push to dataLayer:

// Page view
analytics.subscribe("page_viewed", (event) => {
  window.dataLayer.push({
    event: "page_viewed",
    timestamp: event.timestamp,
    client_id: event.clientId,
    url: event.context.document.location.href,
    page_title: event.context.document.title,
  });
});

// Product view
analytics.subscribe("product_viewed", (event) => {
  window.dataLayer.push({
    event: "product_viewed",
    timestamp: event.timestamp,
    client_id: event.clientId,
    url: event.context.document.location.href,
    product_id: event.data?.productVariant?.product?.id,
    product_title: event.data?.productVariant?.title,
    product_sku: event.data?.productVariant?.sku,
  });
});

// Add to cart
analytics.subscribe("product_added_to_cart", (event) => {
  window.dataLayer.push({
    event: "product_added_to_cart",
    timestamp: event.timestamp,
    client_id: event.clientId,
    url: event.context.document.location.href,
    price: event.data?.cartLine?.merchandise?.price?.amount,
    product_title: event.data?.cartLine?.merchandise?.product?.title,
    quantity: event.data?.cartLine?.quantity,
    total_cost: event.data?.cartLine?.cost?.totalAmount?.amount,
  });
});

// Checkout complete
analytics.subscribe("checkout_completed", (event) => {
  window.dataLayer.push({
    event: "checkout_completed",
    timestamp: event.timestamp,
    token: event.data?.checkout?.token,
    client_id: event.clientId,
    email: event.data?.checkout?.email,
    orderId: event.data?.checkout?.order?.id,
    currency: event.data?.checkout?.currencyCode,
    value: event.data?.checkout?.totalPrice?.amount,
    tax: event.data?.checkout?.totalTax?.amount,
    shipping: event.data?.checkout?.shippingLine?.price?.amount,
  });
});

Shopify → GA4 Event Mapping

Shopify Event	GTM Trigger	GA4 Event
page_viewed	Custom Event	page_view
product_viewed	Custom Event	view_item
product_added_to_cart	Custom Event	add_to_cart
product_removed_from_cart	Custom Event	remove_from_cart
cart_viewed	Custom Event	view_cart
checkout_started	Custom Event	begin_checkout
checkout_address_info_submitted	Custom Event	add_shipping_info
payment_info_submitted	Custom Event	add_payment_info
checkout_completed	Custom Event	purchase

Migrating Legacy dataLayer.push Code

Legacy method (in theme.liquid):

<script>
  dataLayer.push({ event: 'email_signup', email: customer.email });
</script>

New method (using Shopify.analytics.publish):

<!-- In theme.liquid -->
<script>
  Shopify.analytics.publish('email_signup', { email: customer.email });
</script>

// In custom pixel
analytics.subscribe("email_signup", (event) => {
  window.dataLayer.push({
    'event': 'email_signup',
    'email': event.customData.email,
  });
});

GTM Trigger Setup

Create a Custom Event trigger in GTM for each Shopify event:

1. Triggers → New → Custom Event
2. Event name: checkout_completed (or relevant event)
3. This trigger fires on: All Custom Events
4. Connect the tag to this trigger

GA4 Tag Configuration

Tag Type: Google Analytics: GA4 Event
Measurement ID: G-XXXXXXXX
Event Name: purchase (or relevant GA4 event)

Event Parameters:
- transaction_id: {{DLV - orderId}}
- value: {{DLV - value}}
- currency: {{DLV - currency}}
- items: {{DLV - items}}

---

noscript Tags

Each GTM container requires a noscript tag:

<!-- GTM Container 1 noscript -->
<noscript><iframe src="https://www.googletagmanager.com/ns.html?id=GTM-XXXX"
height="0" width="0" style="display:none;visibility:hidden"></iframe></noscript>

<!-- GTM Container 2 noscript -->
<noscript><iframe src="https://www.googletagmanager.com/ns.html?id=GTM-YYYY"
height="0" width="0" style="display:none;visibility:hidden"></iframe></noscript>

---

Testing and Debugging

GTM Preview Mode

Preview Mode should be activated separately for each container:

1. Start Preview for GTM-XXXX
2. Start Preview for GTM-YYYY in a separate tab
3. Check both debug panels on the target page

Tag Assistant

Use Google Tag Assistant to check:

• Which tags are firing
• dataLayer contents
• Consent state

Debug with Container ID

// Check specific container's state
console.log(window.google_tag_manager['GTM-XXXX']);
console.log(window.google_tag_manager['GTM-YYYY']);

---

Common Errors and Solutions

Error	Cause	Solution
Triggers not working	Different dataLayer names	Use single dataLayer
Data loss	Using dataLayer = []	Use window.dataLayer = window.dataLayer || []
Duplicate events	Generic event names	Use namespaced names
PII leakage	Sensitive data in shared dataLayer	Process data server-side
Consent violation	Missing Consent Mode	Integrate Consent Mode v2

---

FAQ

Can I use multiple GTM containers?

Yes, officially supported since 2015. However, all containers must share the same dataLayer.

Should I use different dataLayers for each GTM?

No. Google’s current recommendation is for all containers to use a single dataLayer. Different dataLayers can cause trigger issues and data loss.

Can I rename the dataLayer?

Technically possible but not recommended. Renaming should only be considered for very specific edge-cases.

Is Consent Mode v2 mandatory?

For sites subject to GDPR, CCPA, and similar regulations, yes. Also required for Google Ads conversion tracking.

When should I use Server-side GTM?

Recommended for performance-critical projects, when PII processing is needed, and for high-traffic sites affected by ad-blockers.

How do I migrate Universal Analytics events to GA4?

Convert Enhanced Ecommerce schema to GA4 event schema. Mappings like addToCart → add_to_cart, products → items are required.

How do I install GTM on Shopify?

Install as a Custom Pixel. Go to Settings → Customer events → Add custom pixel. Add GTM code without HTML tags and use analytics.subscribe() to listen to Shopify events and push to dataLayer.

What are Shopify sandbox limitations?

Limited direct DOM access, no document.cookie access, restricted localStorage/sessionStorage. Some GTM features (scroll tracking, click tracking) may not work inside the sandbox.

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Key Takeaways

1. Use single dataLayer — All GTM containers must share the same dataLayer
2. Safe instantiation — Use window.dataLayer = window.dataLayer || []
3. Specific event names — Prefer namespaced, unique event names
4. Consent Mode v2 — Mandatory for GDPR/CCPA compliance
5. GA4 schema — Universal Analytics deprecated, migrate to GA4 events
6. Consider server-side — Evaluate server-side GTM for performance and privacy
7. Test, test, test — Validate every change with Preview Mode and Tag Assistant

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Resources

• Google Tag Manager Data Layer Documentation
• Consent Mode Implementation Guide
• GA4 Ecommerce Events Reference
• Server-side Tagging Guide
• Shopify GTM Custom Pixel Tutorial