Google Publisher Tag (GPT) is an asynchronous JavaScript library provided by Google that enables publishers to implement and manage advertising on their websites. It allows for the definition of ad slots, configuration of ad targeting, and display of ads served from Google Ad Manager. When loaded, the script communicates with Google's ad servers to fetch relevant advertisements based on various criteria, including user context, page content, and publisher-defined targeting. GPT supports features like single request architecture (SRA) to request multiple ad slots simultaneously, responsive ad units for different screen sizes, and impression-level controls such as custom creative rendering and ad refresh. It ensures ads are loaded efficiently without blocking page content, improving user experience.