Creating a multi-page React app with Gatsby involves setting up an application, adding a router, and defining routes for different pages. The `gatsby-node.js` file can be used to create page-holders and enable hyperlinks between views. Client-side routing libraries like react-router-dom allow developers to register URLs and their corresponding components in the DOM tree. SPAs get data by making smaller requests for only the data needed, whereas traditional websites request the entire HTML document again. The Finnhub API can be used to fetch stock data, which requires an account on RapidAPI. By using Gatsby.js, developers can create a multi-page React app that combines the benefits of both SPAs and MPAs, including enhanced SEO and fast dynamic page routing.