Google Publisher Tag (GPT) is an ad tagging library provided by Google that allows publishers to define ad slots on their webpages, make ad requests to Google Ad Manager, and render ads. This script facilitates the asynchronous loading of ads, which improves page performance by not blocking content rendering. It also enables advanced ad serving features such as key-value targeting, responsive ad units for different screen sizes, single request architecture (SRA) to request multiple ad slots at once, and management of ad refreshes. GPT also includes functionalities for collecting viewability metrics and managing consent in compliance with privacy regulations. The script communicates with Google's ad servers to fetch and display relevant advertisements.