GUERLAIN: Restoring Absolute Trust in Brand Analytics with Google Tag Gateway

Overcoming Marketing Signal Loss and Measurement Hurdles

Part of the LVMH Group, Guerlain is a global leader in the luxury cosmetics market. Recently, the brand faced critical signal loss from its marketing campaigns. Signal loss is what happens when marketing teams get less accurate data from users, which limits their ability to track various behaviors and determine if a campaign is effective or not. This affects a brand’s ability to target audiences and attribute sales to touchpoints accurately, and can lead to wasted ad spend as well.

In Guerlain’s case, the loss was largely driven by tightening privacy regulations and Safari’s strict Intelligent Tracking Prevention (ITP) restrictions (which severely impact mobile-heavy Safari traffic by deleting client-side cookies within 24 to 72 hours),

To provide more resilient signals for its media teams to optimize performance, Guerlain needed to transition to a first-party server-side tracking architecture through Google Tag Gateway, or GTG; however, the brand needed to determine the exact ROI of this solution. Standard "before-and-after" comparisons were impossible due to external traffic fluctuations like seasonality and promotional media campaigns, meaning Guerlain needed a way to isolate and calculate the true conversion uplift based on real-time figures.

Transitioning to First-Party Server-Side Tracking to Confirm Accurate Business Value Measurement

To overcome tracking blockers, Google and fifty-five transitioned Guerlain from its legacy client-side stack, where multiple scripts fire directly from the page under high data loss risk, to a robust first-party server-side architecture. They deployed GTG alongside Server-side Google Tag Manager (sGTM); this architecture converts the data flow into privacy-compliant first-party data that restores lost signals to Google Analytics and Google Ads.

To isolate the precise business value of GTG, fifty-five engineered a parallel double-tracking methodology. They deployed a secondary client-side GTM container as a standard third-party "control group" to enable the real-time comparison of data collection efficiency. Set-up without the need for technical assistance from development teams, this off-site deployment can be activated and deactivated at will.

Quantified Business Impact and Conversion Uplift from Improved Data Recovery

The two-month parallel test quickly proved the massive value of Google's first-party architecture, delivering a +16% uplift in GA4 sessions, a +9% uplift in GA4 purchases and revenue, and a +8% uplift in Google Ads conversions and ROAS. By recovering previously "invisible" customer conversions, this implementation successfully restored missing signals to feed Google's advanced AI and bidding algorithms.

By replacing browser-degraded gaps with high-quality, privacy-compliant data, Guerlain can now make data-based decisions with complete confidence in its web analytics.