Gtest and gmock in below code snippet though error

How to achieve gtest or gmock for the private and protected member function. I am new to gtest and gmock. Below is the code for which i need to do gtest or gmock along with my attemp.

constexpr static char _session[]{"S_ID"};

typedef struct {
    int  session;

} Session;

typedef std::function<void(const Session &)> SessionCallback_t;

class Service : public ParentService {
public:
    Service();

    void registerCallback(const SessionCallback_t & callback);

protected:
    virtual void notifyHandler(const Json::Value & data) override;
    virtual void notifyState();
private:
   
    Session mSession;
    SessionCallback_t mCallback;

    void jsonParse(const Json::Value & json_data);
};

My Attemp which doesn't compile

class TestService : public Service {
        public:
        TestService(): Service() {
        }
        bool registerCallback(const SessionCallback_t & cb) {
           // how to achive this?
              
        }


};


class MyTestService : public ::testing::Test {
    
protected:
  virtual void SetUp() {
  }

  virtual void TearDown() {
    
  }
};

 

TEST_F(MyTestService , registerCallbackTest) {
      TestService service;
       EXPECT_TRUE(service.registerCallback(SessionCallback_t));
    
}   

I am stuck with the below interface

1.registerCallback()

2.notifyHandler()

3.notifyState()

4.jsonParse()

Please though some light to proceed further.