Dgraph.app and Amazon's AppSync do this already from a backend but I am looking for something that can run locally. At a high level, the engine is a GraphQL ORM for data in SQLite or WatermelonDB.
Alternatives are https://github.com/graphile/postgraphile or dgraph as you mentioned. Hasura is working on support for sqlite, so you may have some blockers there, you can also look into the Prisma engine which has GQL as an intermediate (for resolvers, for example)