GraphQLZero is described as 'free, online GraphQL API that you can use to get fake data from a real backend while testing or prototyping your app. It is inspired and powered by JSONPlaceholder, serving the same dataset but in the form of a GraphQL server' and is an app. There are seven alternatives to GraphQLZero for a variety of platforms, including Self-Hosted solutions, Online / Web-based, Linux, SaaS and GraphQL. The best alternative is Prisma GraphQL API, which is both free and Open Source. Other great apps like GraphQLZero are GraphCMS, PostGraphile, GraphQL Playground and GraphQl Editor.
GraphQL Playground is an IDE for better development workflows (GraphQL Subscriptions, interactive docs & collaboration). You can download the Electron-based desktop app or use the web version at graphqlbin.com.
Squidex is an open source headless CMS and content management hub. In contrast to a traditional CMS Squidex provides a rich API with OData filter and Swagger definitions. It is up to you to build your UI on top of it. It can be website, a native app or just another server.