Google Publisher Tag (GPT) is an ad tagging library for publishers to define ad inventory, initiate ad requests, and render demand. It is used to request ads from Google Ad Manager and display them on web pages. The script facilitates the integration of various ad formats, including display, native, and video ads, into a website's content. GPT provides functionalities for building ad requests with specific parameters, managing multiple ad slots on a single page, and ensuring ads are displayed correctly and efficiently. It supports responsive ad units, single request architecture (SRA) to improve page performance by fetching all ads for a page in one request, and other advanced ad serving features. This script primarily interacts with Google's ad servers to fetch and display targeted advertisements based on user context, publisher settings, and privacy signals.