Have you created protocol definitions for gRPC yourself?

Yes, I have created protocol definitions for gRPC myself. This is usually done using a .proto file, where services and messages are described. For example:

syntax = "proto3";

package example;

service UserService {
  rpc GetUser (UserRequest) returns (UserResponse);
}

message UserRequest {
  int32 id = 1;
}

message UserResponse {
  string name = 1;
  int32 age = 2;
}

After that, the appropriate interfaces and structures are generated using protoc and plugins for Go, which can be used in the code to implement the service and client.