Here’s an article on how to use Gatsby in a Phoenix (Elixir) app. It looks like the basic concept could be applied to using any static site generator (Gatsby, Nuxt, Gridsome, Hugo, etc.) with any dynamic web framework (Phoenix, Rails, Flask, Django, etc.).
Basically, create the static site inside your assets source directory, build the static site, and symlink the static site’s output directory to your framework’s public directory.