Google Publisher Tag (GPT) is a JavaScript library provided by Google that enables websites to define and render ad slots from Google Ad Manager. It allows publishers to manage their ad inventory, implement targeting, and control ad display on their web pages. GPT loads asynchronously, minimizing its impact on page load times, and fetches ads within a Single Request Architecture (SRA) to improve performance and prevent competitive exclusion issues. It also supports responsive ad units, allowing ads to adapt to different screen sizes and devices. The script communicates with Google's ad servers to request ads based on defined criteria (e.g., ad unit, targeting keys, user context) and then renders the received ad creative in the specified ad slots. It handles ad refresh, impression tracking, and click tracking, providing detailed reporting capabilities for publishers. GPT is a client-side solution that leverages cookies and other identifiers to facilitate ad personalization, frequency capping, and performance measurement.