The Google Publisher Tag (GPT) script is a JavaScript library provided by Google that allows publishers to implement and manage ads from Google Ad Manager on their websites. It's a client-side script that handles the entire ad lifecycle, from defining ad slots and requesting ads from Google's ad servers to rendering the ad creatives and managing ad events. GPT is designed for asynchronous ad loading, which means ads load independently of page content, preventing any delays in page rendering. It supports Single Request Architecture (SRA) to request multiple ad units simultaneously, improving performance and accuracy of competitive exclusions. The script also provides robust targeting capabilities, allowing publishers to target ads based on various criteria like user demographics, page content, and custom key-values. It interacts with Google's ad serving infrastructure to fetch the most relevant advertisements. The script manages ad refreshing, viewability measurement, and integrates with Google's broader advertising and analytics ecosystem. It also handles various ad formats including display, video, and native ads, and is essential for publishers utilizing Google Ad Manager for monetization.