The Google Publisher Tag (GPT) is an ad tagging library provided by Google that allows publishers to define ad slots on their webpages and programmatically request ads from Google Ad Manager. It loads asynchronously, which helps to improve page load times by not blocking the rendering of other page content. GPT handles the communication with Google's ad servers, retrieves ad creatives, and renders them into the defined ad slots. It supports various ad formats, targeting capabilities, and responsive design, ensuring ads display correctly across different devices and screen sizes. Publishers use GPT to manage their ad inventory, implement ad policies, and integrate with demand sources like Google AdSense and third-party ad exchanges, enabling precise control over ad delivery and monetization.