Gatsby, traditionally known for building static sites, is equally adept at creating dynamic web applications, combining the benefits of static site generation and app-like functionality. By leveraging client-side JavaScript and React, Gatsby enables features such as dynamic data fetching, user authentication, and client-only routes, making it suitable for applications like Gmail and Twitter, which epitomize modern web apps. The framework's built-in performance optimizations, such as static rendering and route-based code splitting, enhance the user experience by making apps appear faster. Gatsby's modular architecture allows for the easy integration of plugins and components, like gatsby-image, which streamline development by handling complex tasks such as image optimization. The use of an app shell pattern, enabled by plugins like gatsby-plugin-offline, allows parts of the application to load instantly while dynamic content is fetched asynchronously, offering a seamless and efficient user experience. This capability is demonstrated in projects like Gatsby Mail, which highlight Gatsby's potential for creating rich, interactive web applications by integrating various functionalities, including GraphQL and service workers.