What is NextJS and Gatsby?
NextJS and Gatsby are both React frameworks that support server-side-rendering and static-site-generation. Both have big community's and around 10k forks on GitHub.
Search Engine Optimization
Search Engine Optimization or SEO is known as how well Google and other browsers will show your site on searches. Create-React-App uses Client Side Rendering and then browsers will not be able to see the vast majority of what your site is about and then does not give it great SEO.
NextJS and Gatsby Solved this by introducing SSR or Server Side Rendering. This means once your code is uploaded the server will generate the static HTML templates and then serve those to the client. Once served the site will become fully reactive with client side rendering.
Community
The Gatsby Community has over 2000 plugins with officially supported and recommended ones available at creation. This can increase productivity and decrease the speed it takes to build a site. An important note about Gatsby is that it uses GraphQL for Static files. This can be more confusing for beginners but is efficient.
NextJS goes with a more officially supported plugins with official Image optimization, Meta, Analytics, and Authentication. NextJS also has a official hosting provider, Vercel which is intuitive and easy to use and has Built in Module CSS bundling.
Production
Both frameworks have similar routing, both with fast refresh and little baseline configuration. They are both Open source and have large community support
NextJS is used by major companies such as Netflix, Twitch, Starbucks, Uber, Hulu and GitHub. NextJS has a considerable amount of more jobs available compared to Gatsby and is typically used more widely.
Gatsby is also used by major companies such as PayPal, Apollo, Spotify, IBM, and Figma even though it is less than NextJS. Gatsby also has its own cloud platform and its own CLI.
In Production Gatsby sites do have a major problem. with server side rendering in sites that are massive in production such as 100k pages GraphQL queries take a considerable amount of time and the Gatsby build time will take hours. This is less of a problem with smaller sites but with sites with content frequently updating Gatsby does not shine. However NextJS does not suffer from this issue so it is the preferred framework among major companies.
Conclusion
While Gatsby is great NextJS has an edge in the real world and is my preferred framework. As it is officially supported by Vercel and is highly used for major sites, it is the framework I would recommend for most sites. However there are still scenarios where Gatsby can perform better that NextJS.
That sounds really cool. I like how you first tell people what they are incase they don't know. You did a good job explaining.