This playground will walk you through a series of GraphQL queries, each designed to demonstrate a particular feature of GraphQL. You'll be querying the real schema used on graphql-reference example so feel free to experiment and poke around the innards of the site! You can also open the Codesandbox version (Direct link to GraphiQL) of it.
You'll be using GraphiQL, an interactive editor you can also use while building your Gatsby site.
For more information, read about why Gatsby uses GraphQL.