I used to have this information stored on a larger gatsby web project from my predecessor, which I have taken over. Since then, I abandoned it and switched to wordpress (currently exploring the idea of headless wordpress design – which might lead to an interface between gatsby and wordpress). So I leave here some raw information as a reference.
What is Gatsby js?
GatsbyJS – free static site generator based on React
Themes vs. starters
Starters are pre-defined sites, which you can use to start new site easy and fast. Themes are like templates. Design and/or functionality, or both. Only Themes can be installed as a npm package.
How to customize them?
You can forking a repo, direct file editing (if you using a starter), or hiding components.
Here are some free resources!