At first glance, Gatsby is a static site generator. It turns markdown into web pages, and serves them from a static CDN. But look behind the scenes, and Gatsby is so much more. It's an ecosystem of plugins. It's an application framework. And it's a great way to get started writing your first PWA.
You can find the full instructions on getting started with Gatsby. What we will step through here is an abbreviated version designed to get your first PWA running.