The _gcl_au cookie is a first-party, advertising-focused cookie set by Google Tag Manager (and often used by Google Ads/AdSense) to store and track conversions, such as purchases or sign-ups. It measures ad performance by storing ad click information, allowing marketers to attribute conversions to specific ad campaigns.Key Details About _gcl_auPurpose: It functions similarly to a conversion linker, ensuring that conversion data is properly attributed back to the ad that drove traffic, even if the conversion happens later or on a different page.