Next.js is the more common of the two in our index: it runs on 4,680 websites against 214 for Gatsby, and 1 sites use both. Counts come from real scans, not vendor claims.
Gatsby is a React-based open-source framework with performance, scalability and security built-in.
Next.js is a React framework for developing single page Javascript applications.
1 site runs both Gatsby and Next.js. That is 0% of the sites using Gatsby, the smaller of the two.
Want to know which one a particular site uses? Look it up and see its whole stack, or compare two sites side by side.