Google Publisher Tag (GPT) is a client-side ad tagging library that enables publishers to define ad slots on their webpages, build ad requests, and render ads from Google Ad Manager. It manages the asynchronous loading of ads, ensures that ads are displayed responsively across different devices, and supports advanced targeting features like key-values and audience segments. The script interacts with Google's ad servers to fetch relevant ad creatives, handles competitive exclusions, and integrates with programmatic demand sources, including header bidding, to maximize ad revenue for publishers. GPT also plays a crucial role in managing the ad lifecycle, including viewability tracking and interaction reporting, which are essential for ad performance measurement and billing. It is designed to optimize page load times by loading ads efficiently without blocking page content.