Google Publisher Tag (GPT) is a JavaScript ad tagging library that allows publishers to define ad slots on their pages and request ads from Google Ad Manager. It dynamically builds ad requests and renders ads on the webpage. The script loads asynchronously, meaning it does not block the rendering of other content on the page, which improves page load times. It supports responsive ad units, allowing ads to adapt to different screen sizes and devices. GPT can also integrate with programmatic advertising methods, including header bidding, to maximize ad revenue. It manages ad lifecycle, including fetching ad creatives, rendering them safely within iframes, and handling ad refreshes. It also provides functionalities for frequency capping, competitive exclusions, and key-value targeting, ensuring a better user experience and effective ad delivery.