I have a file name .env.development in the root folder. I had install env-cmd as dev dependencies when I start the server
> npm run develop
In Windows. First create file .env.development Then add in package.json: "develop": "env-cmd -f .env.development --fallback gatsby develop" and: npm run develop attached: https://css-tricks.com/using-graphql-playground-with-gatsby
.env.development
"develop": "env-cmd -f .env.development --fallback gatsby develop"
npm run develop