Author here. I love and use both. For static sites, Zola is super nice: it's fast, has a great community, no thrills, and it gets better all the time. The config options are sensible and the templating engine is easy to learn. Sites I'm maintaining with Zola: https://endler.dev/, https://hello-rust.show/

For dynamic content, I love Gatsby. It can read from whatever you throw at it: Files (Markdown, plaintext, YAML), APIs (e.g. Github),... I'm not a frontend dev so my React knowledge is quite limited, but it's enough for simple components. Sites I'm maintaining with Gatsby: https://analysis-tools.dev/

Creator of Zola so a bit biased. Zola actually comes with an automatic search engine built-in that you can see on https://www.getzola.org/

I haven't used Gatsby.js as doing React/GraphQL for my static site isn't really something I'm interested in. If your site is very dynamic/pulls from various remote sources or just want to use React it's better than Zola.

Just started using Zola recently. Early, but after comparing with several other engines it seemed the best suited to my application. So far I'm happy with it.