1. 1. Feature Set: Gitbook and Sphinx have key differences when it comes to their feature set. Gitbook provides a robust and user-friendly interface for creating, editing, and publishing documentation, including support for versioning, multiple languages, and collaboration features. On the other hand, Sphinx is a documentation generator that uses reStructuredText as its markup language, offering more advanced features for documenting code and projects, such as automatic API documentation generation, customizable themes, and support for large documentation projects.
2. 2. Markup Language: Another significant difference lies in the markup language used by Gitbook and Sphinx. Gitbook uses Markdown as its primary markup language, which is a simple and lightweight syntax that is easy to learn and write. Sphinx, on the other hand, utilizes reStructuredText, which offers more powerful capabilities for documenting complex software projects, including support for cross-references, inline code highlighting, and linking to external resources. This difference in markup language can influence the ease of use and flexibility of the documentation tools.
3. 3. Output Formats: Gitbook and Sphinx also diverge in terms of the output formats they support. Gitbook primarily generates HTML-based documentation which can be easily viewed on any web browser. Gitbook also offers the ability to export the documentation as PDF and eBook formats. On the other hand, Sphinx supports a wide array of output formats, including HTML, PDF, ePub, man pages, and even LaTeX, allowing for more flexibility in distributing and publishing the documentation across different platforms and mediums.
4. 4. Customization and Theming: Gitbook provides a range of customizable themes and templates that allow users to personalize the look and feel of their documentation. Users can easily apply themes and modify CSS styles to match their branding or design preferences. In contrast, Sphinx offers extensive theming options, including the ability to create custom themes, providing more control and flexibility for tailoring the visual appearance of the documentation to specific needs. Sphinx also provides an ecosystem of plugins and extensions that further enhance the customization options.
5. 5. Hosting and Collaboration: Gitbook offers a cloud-based hosting solution where the documentation can be published and accessed online, making it easy to share and collaborate with multiple users. Gitbook also provides features such as versioning, comments, and collaboration tools to facilitate teamwork and community involvement. On the other hand, Sphinx does not offer a dedicated hosting platform and requires separate hosting or deployment options, such as serving the documentation through a web server or integrating it into existing websites. Collaboration features like version control and commenting may need to be implemented using external tools or plugins.
6. 6. Learning Curve and Community: Gitbook has a lower learning curve due to its simplicity and intuitive user interface, making it more accessible to beginners. It also has a growing community of users, providing support, documentation, and plugins. Sphinx, on the other hand, has a steeper learning curve, especially for those unfamiliar with reStructuredText, but offers more advanced features for large-scale and technical documentation projects. Sphinx has an established and active community, comprising developers, contributors, and a vast ecosystem of plugins, providing a wealth of resources and support for users.

In Summary, Gitbook and Sphinx differ in their feature set, markup language, output formats, customization options, hosting, collaboration capabilities, and learning curve.