public interface MessagesApi {
    @GET("messages1.json")
    Call<List<Message>> messages();
}

public class Message {
        private long id;
        private long time;
        private String text;
        private String image;

        // getters and setters
}

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Retrofit retrofit = new Retrofit.Builder()
                .baseUrl("https://rawgit.com/startandroid/data/master/messages/")
                .addConverterFactory(GsonConverterFactory.create())
                .build();

        MessagesApi messagesApi = retrofit.create(MessagesApi.class);

        Call<List<Message>> messages = messagesApi.messages();

        messages.enqueue(new Callback<List<Message>>() {
            @Override
            public void onResponse(Call<List<Message>> call, Response<List<Message>> response) {
                if (response.isSuccessful()) {
                    Log.d("tag", "response " + response.body().size());
                } else {
                    Log.d("tag", "response code " + response.code());
                }
            }

            @Override
            public void onFailure(Call<List<Message>> call, Throwable t) {
                Log.d("tag","failure " + t);
            }
        });
    }
}

if (response.isSuccessful()) {
   if(response.body() != null)
      for(Message message: response.body()) {
         // Тут имеем доступ к элементу списка сообщений
         Log.d("tag", "Message [id=" + message.getId() + "], text=" + message.getText());
   }
}