In the #100DaysOfGatsby challenge, participants are encouraged to transform their Gatsby sites into Progressive Web Apps (PWAs), enhancing functionality for offline use and poor internet connectivity. Gatsby, often recognized as a static site generator, is highlighted for its capabilities in creating PWAs, which offer features like offline mode, home screen installation, and push notifications, similar to native mobile apps. To achieve this, users are guided to implement plugins such as gatsby-plugin-manifest and gatsby-plugin-offline, enabling these advanced features. The challenge is designed for both beginners and experienced developers, with support available through the Gatsby community on various platforms like Discord and Reddit.