I don't know if setLogLevel() will return in the final 2.0 version of Retrofit but for now you can use an interceptor for logging.
A good example can found in OkHttp wiki: https://github.com/square/okhttp/wiki/Interceptors
OkHttpClient client = new OkHttpClient();
client.interceptors().add(new LoggingInterceptor());
Retrofit retrofit = new Retrofit.Builder()
.baseUrl("http://www.yourjsonapi.com")
.addConverterFactory(GsonConverterFactory.create())
.client(client)
.build();