I started to look into building a POC integrating Shopify with GatsbyJS but I found an excellent starter already exists. Here are the instructions to run the demo.
If you don’t have the gatsby-cli installed, now is the time. Follow instructions at https://www.gatsbyjs.org/tutorial/part-zero/
Now you can run the demo:
gatsby new gatsby-shopify-starter https://github.com/AlexanderProd/gatsby-shopify-starter
Open a browser to http://localhost:8000 to see the demo.
The demo leans on a plugin called gatsby-source-shopify to get the product information from Shopify API. See gatsby-config.js for the plugin setup, gatsby-node.js for the product page generation setup, and src/templates/ProductPage/index.js for the product template.
The User, Cart and Checkout interaction is defined in src/providers/ContextProvider.js using the shopify-buy package.