In the final phase of the #100DaysOfGatsby coding challenge, participants are tasked with creating a Gatsby Theme to facilitate scalable growth for a fictional company named AudioCORE. This involves developing a pre-built, reusable package that simplifies the setup and configuration of Gatsby sites. Participants have access to various resources, including documentation, tutorials, and community support, to assist in their theme development. The challenge encourages sharing progress on forums, attending livestreams for best practices, and publishing the completed theme to the npm library. Successful completion marks the end of the initiative, unlocking new skills and offering opportunities for further learning and engagement with the Gatsby community.