Google Publisher Tags (GPT) Integration

Category: Ad Serving Status: Production Type: First-Party Ad Tag Delivery

Overview

The GPT integration delivers Google Publisher Tags via the publisher's domain by proxying GPT's script cascade in first-party context. This avoids cross-origin script loads, improving performance and reducing friction with ad blockers and Intelligent Tracking Prevention.

What is GPT?

Google Publisher Tags (GPT) is the JavaScript library publishers use to define and render ad slots served by Google Ad Manager. GPT loads scripts in a cascade:

1. gpt.js -- the thin bootstrap loader
2. pubads_impl.js -- the main GPT implementation (~640 KB)
3. pubads_impl_*.js -- lazy-loaded sub-modules (page-level ads, side rails, etc.)
4. Auxiliary scripts -- viewability, monitoring, error reporting

All of these are served from securepubads.g.doubleclick.net.

How It Works

  Publisher HTML
  │
  ├─ <script src="securepubads.g.doubleclick.net/tag/js/gpt.js">
  │   ↓ (attribute rewriter)
  │   <script src="publisher.com/integrations/gpt/script">
  │
  ├─ Server fetches gpt.js from Google, serves it verbatim
  │
  ├─ Client-side shim intercepts dynamic script insertions
  │   ↓ (script guard)
  │   securepubads.g.doubleclick.net/pagead/…
  │   → publisher.com/integrations/gpt/pagead/…
  │
  └─ Server proxies cascade scripts from Google, serves verbatim

There are three layers:

1. HTML attribute rewriting (server-side) -- Rewrites src/href attributes on the initial gpt.js <script> tag to the first-party endpoint /integrations/gpt/script.

2. Script proxy (server-side) -- Fetches scripts from Google and serves them through the publisher's domain. Script bodies are served verbatim with no modification.

3. Client-side shim -- A script guard (script_guard.ts) uses six interception layers -- document.write interception, HTMLScriptElement.prototype.src property descriptor, setAttribute patch, document.createElement patch, DOM insertion patches, and a MutationObserver -- to catch GPT script URLs regardless of how they are set or inserted. The document.write layer is the most critical, as GPT's primary loading path uses document.write to synchronously inject pubads_impl.js into the HTML parser stream. This is the sole mechanism that routes GPT's cascaded script loads back through the proxy.

Configuration

Add GPT configuration to trusted-server.toml:

[integrations.gpt]
enabled = true
gam_attribution_enabled = false
script_url = "https://securepubads.g.doubleclick.net/tag/js/gpt.js"
cache_ttl_seconds = 3600
rewrite_script = true

Configuration Options

Field	Type	Required	Default	Description
enabled	boolean	No	true	Enable/disable the integration
gam_attribution_enabled	boolean	No	false	Add fixed page-level ts=true targeting for GAM cohort reporting
script_url	string	No	https://securepubads.g.doubleclick.net/tag/js/gpt.js	URL for the GPT bootstrap script
cache_ttl_seconds	integer	No	3600	Cache TTL for proxied scripts (60--86400s)
rewrite_script	boolean	No	true	Whether to rewrite GPT script URLs in HTML

The environment override TRUSTED_SERVER__INTEGRATIONS__GPT__GAM_ATTRIBUTION_ENABLED works only when gam_attribution_enabled is already present under [integrations.gpt] in the TOML file. The environment overlay cannot create a missing configuration leaf.

Endpoints

• GET /integrations/gpt/script -- Serves the GPT bootstrap script (gpt.js)
• GET /integrations/gpt/pagead/* -- Proxies secondary GPT scripts and resources
• GET /integrations/gpt/tag/* -- Proxies tag-path resources

Successful proxy responses include the X-GPT-Proxy: true header for debugging.

Features

• Full cascade proxying: Every script in GPT's loading chain is served first-party
• Verbatim script delivery: No server-side script modification -- scripts are proxied as-is
• Client-side interception: DOM-level script guard catches all dynamic script insertions
• Configurable caching: Tune TTL per deployment (default 1 hour, range 60s--24h)
• HTML attribute rewriting: Automatic rewrite of src/href attributes in publisher HTML
• Protocol-aware: The client-side shim matches the page's protocol (HTTP for local dev, HTTPS for production)

Client-Side Shim

The GPT integration includes a TypeScript module bundled into the unified TSJS bundle. It provides two capabilities:

Script Guard

The script guard uses six interception layers to catch GPT script URLs regardless of how they are set or inserted into the DOM:

1. document.write / document.writeln -- GPT's primary loading mechanism. When gpt.js loads synchronously, it uses document.write to inject <script src="...pubads_impl.js"> directly into the HTML parser stream. The guard intercepts these calls and rewrites URLs whose hostname is securepubads.g.doubleclick.net inside the HTML string before passing it to the native method.
2. Property descriptor on HTMLScriptElement.prototype.src -- intercepts script.src = url assignments. This catches GPT's async fallback path (used when document.write is unavailable, e.g. after page load or with async scripts).
3. setAttribute patch on HTMLScriptElement.prototype -- catches script.setAttribute('src', url) calls that bypass the property setter.
4. document.createElement patch -- tags every newly created <script> element with a per-instance src descriptor, ensuring coverage even if the prototype-level descriptor cannot be installed.
5. DOM insertion patches on appendChild / insertBefore -- catches scripts and <link rel="preload"> elements whose src/href is already set at insertion time.
6. MutationObserver -- catches elements added via innerHTML, .append(), or other DOM methods, as well as attribute mutations on existing elements.

Intercepts scripts from securepubads.g.doubleclick.net and rewrites them to the first-party proxy.

Command Queue Patch

Takes over googletag.cmd so every queued callback is wrapped before GPT executes it. This enables future hook points for:

• EC ID injection as page-level key-value targeting
• Consent gating of ad requests
• Ad-unit path rewriting for A/B testing

GAM Treatment Attribution

Setting gam_attribution_enabled = true adds the fixed page-level GPT targeting value ts=true. It is applied before publisher GPT initialization and remains for the browser document's lifetime, so initial, lazy, refresh, publisher-owned, and SPA-route requests inherit it unless another targeting consumer clears or overrides the key. The attribution switch is independently controlled and defaults to false, but the GPT integration's enabled master switch must also be true.

This key is distinct from the existing slot-level ts_initial=1 value. ts_initial retains its current cleanup lifecycle; Trusted Server does not clear the page-level ts value during Prebid refresh or SPA cleanup.

For an eligible publisher document whose activation script was not cloned, ts=true means Trusted Server emitted the rewritten document head before the GPT request. It does not prove that the response body completed, that a Trusted Server bid won, or that an impression was caused by treatment. A publisher can copy the activation script with srcdoc or document.write; treat any marker on an unrewritten nested document as contamination, not attribution proof.

Before enabling attribution in a cohort:

1. Complete privacy and CSP review, create the reportable predefined true value in the target GAM network, and verify the chosen GAM reporting surface and billing approval.
2. Audit the short ts key across publisher GPT code, effective Prebid bidderSettings[*].adserverTargeting output (including setTargetingForGPTAsync), the effective creative-opportunity targeting map, and every GAM consumer that can affect eligibility, pricing, protection, or routing. Trusted Server accepts and forwards operator targeting verbatim; it does not reserve, filter, or intercept a slot-level ts key at runtime.
3. With treatment routing stopped, deploy attribution enabled and validate initial, lazy, refresh, publisher-owned, and SPA requests. Confirm every excluded path reports zero marked requests, then save a short paired-report dry run that satisfies the invariants below before starting the cohort.

For reporting, save one exact eligible universe: GAM network, inventory units, routes, formats, time zone, date window, metrics, and all exclusions. Report A is the nonduplicated total for that universe. Report B uses identical filters and metrics plus exactly ts=true. If Enhanced Key-Value reporting is unavailable, unapproved, or incompatible, use an exactly filtered legacy key-value report and never sum its repeated key-value rows. Derive control as A - B, and require 0 <= B <= A for every metric. A violation invalidates the whole report pair; never clamp a negative result. Use the same reporting-latency and invalid-traffic maturation window for both reports.

GAM results are descriptive delivery attribution, not a causal treatment effect. Aggregate monitoring and synthetic/manual samples can detect obvious failures but cannot prove marker completeness on every production request without request-correlated telemetry.

For a normal rollback, first stop and verify new treatment assignment at the router, record a clean reporting boundary, and let already-open documents drain. Exclude the drain interval, then set gam_attribution_enabled = false after marked traffic reaches zero for the agreed interval. An emergency kill may flip the setting immediately, but the affected interval and subsequent drain must be treated as invalid for experiment reporting.

Use Cases

First-Party Ad Delivery

Problem: Third-party script loads from Google's domains are blocked by ad blockers and browser privacy features.

Solution: GPT integration routes all scripts through the publisher's domain, making them indistinguishable from first-party resources.

Local Development

Problem: GPT scripts fail to load or behave differently in local development environments.

Solution: The integration works with both HTTP and HTTPS schemes. When running locally with Viceroy, the client-side shim produces http:// URLs matching the dev server.

Troubleshooting

Scripts Not Loading Through Proxy

Symptoms: Network tab shows requests to securepubads.g.doubleclick.net instead of first-party domain.

Solutions:

• Verify rewrite_script is true in config
• Check that the TSJS bundle with the GPT shim is loaded before GPT
• Inspect console for "GPT guard: installing interception for Google ad scripts" log message

Ads Not Rendering

Symptoms: Ad slots remain empty after proxying.

Solutions:

• Check the proxy responses have 200 status (look for X-GPT-Proxy: true header)
• Verify the script_url config points to the correct GPT endpoint
• Review server logs for upstream fetch failures
• Open GPT Runtime Diagnostics with ?ts_console=1 to see the observed request, render, load, and delivery evidence per slot

Implementation

• Rust: crates/trusted-server-core/src/integrations/gpt.rs
• TypeScript: crates/trusted-server-js/lib/src/integrations/gpt/

Next Steps

• Use GPT Runtime Diagnostics to inspect GPT lifecycle and Trusted Server delivery evidence in the browser
• Review Integrations Overview for comparison with other integrations
• Check Configuration Reference for advanced options
• Learn about First-Party Proxy architecture
• See Google Ad Manager for the planned direct GAM integration