OpenAPI => gRPC

nytimes/openapi2proto: A tool for generating Protobuf v3 schemas and gRPC service definitions from OpenAPI specifications @GitHub

This tool will accept an OpenAPI/Swagger definition (yaml or JSON) and generate a Protobuf v3 

schema and gRPC service definition from it.

gRPC web site
https://grpc.io

OpenAPI and gRPC Side-by-Side - APIs and Digital Transformation - Medium

Comparing OpenAPI with gRPC – Red Hat Services Speak

gRPC with REST and OpenAPI Specification - OpenAPI Initiative