Google Publisher Tag (GPT) is a client-side JavaScript library developed by Google that enables publishers to define ad slots on their web pages and request ads from Google Ad Manager. It facilitates the asynchronous fetching and rendering of advertisements, ensuring that ad loading does not block the rendering of other page content. When a web page loads, the GPT script makes requests to Google Ad Manager servers, which then determine and deliver the most relevant ad creatives based on various parameters like targeting criteria, available inventory, and user data. The library is responsible for displaying these ads within their designated slots, handling responsive ad sizing, ad refreshes, and granular key-value targeting. It's a critical component for publishers using Google Ad Manager to monetize their digital content efficiently and flexibly. GPT works by creating ad slots on the page which are then filled with ads retrieved from Google's ad servers. It supports a Single Request Architecture (SRA) to send all ad slot requests to the server at once, which can improve page performance. The script also provides functionalities for controlling ad behavior, such as collapsing empty ad slots and managing ad unit sizes dynamically to adapt to different screen dimensions and device types. Its primary goal is to provide a robust and high-performance framework for display advertising.