I think you might be wrong in your understanding. Webpack is a bundler, which Gatsby uses. You can use Gatsby as a replacement for Jekyll or Ghost, they do the

I think you might be wrong in your understanding. Webpack is a bundler, which Gatsby uses. You can use Gatsby as a replacement for Jekyll or Ghost, they do the same thing, i.e. output static HTML. However Gatsby uses React and GraphQL to achieve that result. While Jekyll uses markdown files for example.