Spring Cloud Gateway OSS contains a filter to automatically transform a JSON request into to gRPC request transparently for the client. You can configure this filter with the following route configuration:

apiVersion: "tanzu.vmware.com/v1"
kind: SpringCloudGatewayRouteConfig
metadata:
  name: test-gateway-grpc-routes
spec:
  routes:
    - uri: https://test-grpc-server-service
      predicates:
        - Path=/json/hello
      filters:
        - JsonToGrpc=file:proto-file/hello.pb,file:proto-file/hello.proto,HelloService,hello

Since gRPC is based on HTTP/2 it is a good practice to secure the gateway application via TLS. A complete configuration requires configuring both the certificate to connect to the gRPC service and the keystore to expose the gateway HTTP endpoint. If your gateway access is not secured (for example, TLS is treated in another layer like ingress) you can ignore the server.ssl.* properties from the example. In order for the gateway to have access to the protobuf definition files, the public certificate, and, for example, the key store. It can be configured the following way:

apiVersion: "tanzu.vmware.com/v1"
kind: SpringCloudGateway
metadata:
  name: test-gateway-grpc
spec:
  env:
    - name: "spring.cloud.gateway.httpclient.ssl.trusted-x509-certificates"
      value: "file:ssl-files/public.cert"

    - name: "server.ssl.key-store-type"
      value: "PKCS12"
    - name: "server.ssl.key-store"
      value: "file:ssl-files/keystore.p12"
    - name: "server.ssl.key-store-password"
      value: "password"

    - name: "server.http2.enabled"
      value: "true"

  podOverrides:
    containers:
      - name: gateway
        volumeMounts:
          - name: proto-file
            mountPath: /workspace/proto-file
          - name: ssl-files
            mountPath: /workspace/ssl-files
    volumes:
      - name: proto-file
        configMap:
          name: proto-file
          items:
            - key: hello.pb
              path: hello.pb
            - key: hello.proto
              path: hello.proto
      - name: ssl-files
        configMap:
          name: ssl-files
          items:
            - key: keystore.p12
              path: keystore.p12
            - key: public.cert
              path: public.cert