What are some alternatives?
When comparing Assemble and GatsbyJS, you can also consider the following products
Wintersmith - Flexible, minimalistic, multi-platform static site generator built on top of node.js
Jekyll - Jekyll is a simple, blog aware, static site generator.
Metalsmith - An extremely simple, pluggable static site generator.
Nikola - Nikola is s static site generator tool written in Python.
Nanoc - A static-site generator written in Ruby
Hexo - A fast, simple & powerful blog framework, powered by Node.js