Surprised they didn’t try Eleventy. I was just reading how a Mozilla team converted their extension documentation to it from Jekyll because it has support for Jekyll’s template language via a plugin.
Compared to 11ty, I would call Gatsby a bloated framework and that's how the developers also describe it "much more than just a SSG". Gatsby is a hidden minefield though, maybe my use cases weren't aligned but I didn't get why it's got so much hype for trying to find the problems that it would become a solution to.
https://hacks.mozilla.org/2020/10/to-eleventy-and-beyond/