TLDR; Gatsby is good for react portfolio sites, but worse than NextJs for web apps and much slower to work and more of a hassle than Web Builders/CMS for brochure/primarily static sites.
I was thinking the same thing, low attack vector, faster, static site. Looked at grav, and Jekyll, and the rest.
I came to the conclusion that these are a solution in search of a problem. Impossible for the average of expert WP person to get into. And to hire a consultant, is cheaper and better to upgrade your hosting, and for the price of a consultant just to get a site set up, (small biz) you could get a bare metal server. Plus in the end you have a very bland website made with markup.