The Google Publisher Tag (GPT) is an ad tagging library for web pages that enables publishers to define ad inventory, initiate ad requests, and render ads. It is used by publishers to make requests to Google Ad Manager and display ads on their websites. GPT allows for advanced ad serving features such as single request architecture, asynchronous ad loading, and dynamic ad unit definitions. It operates by inserting iframes into the page where ads are displayed, ensuring that ad content is isolated from the main page content. GPT handles the communication with Google's ad servers, determines which ads to show based on targeting criteria, and manages the rendering process. It is a critical component for monetizing websites through Google Ad Manager. It also supports various ad formats, detailed targeting options, and frequency capping.