It is easy to wrap your existing REST API into a GraphQL API. The advantages of these are many:
you can incrementally adopt GraphQL in your existing infrastructure,
you get added benefits over REST APIs like type safety and more readability,
you get a unified API layer for all your APIs.
With this guide, you can get started with your GraphQL wrapped REST API in minutes. You can then add this GraphQL service as a Remote Schema in Hasura for automatic schema-stitching so that you get a whole bunch of auto-generated APIs and your own custom logic together in one interface.