The #100DaysOfGatsby challenge encourages developers to optimize their websites for search engines using Gatsby, a framework known for its speed and efficient SEO capabilities. Gatsby's fast-loading sites provide a ranking advantage on Google, as they prefetch pages and create navigation that search engine spiders can easily crawl. This can help reduce bounce rates and improve site engagement, which are significant factors in search engine rankings. The challenge emphasizes going beyond Gatsby's default SEO features by customizing site descriptions, adding metadata for better previews, and utilizing sitemaps. Participants can find support through the Gatsby community and various online platforms, ensuring they can effectively enhance their site's search engine performance.