Google Publisher Tag (GPT) is a JavaScript library used by publishers to integrate Google Ad Manager advertisements into their websites. It enables the definition of ad slots, sends ad requests to Google's ad servers, and renders the ad creatives on the page. The script supports various ad formats, targeting options, and advanced features such as competitive exclusion and asynchronous ad loading, which contributes to improved page performance and user experience. GPT also plays a crucial role in ad viewability tracking and ensures that ads are displayed in compliance with established advertising standards. It handles the lifecycle of an ad, from fetching to displaying and tracking interactions, providing a robust framework for managing digital advertising inventory.