Retrofit 2 example tutorial but GsonConverterFactory display error "Cannot resolve symbol"

I'm trying to follow Retrofit's 2 tutorial, but on this part of the code there is a GsonConverterFactory that displays error Cannot resolve symbol:

public class ServiceGenerator {

    public static final String API_BASE_URL = "http://your.api-base.url";

    private static OkHttpClient httpClient = new OkHttpClient();
    private static Retrofit.Builder builder =
            new Retrofit.Builder()
                    .baseUrl(API_BASE_URL)
                    //THIS IS THE LINE WITH ERROR!!!!!!!!!!!!
                    .addConverterFactory(GsonConverterFactory.create());

    public static <S> S createService(Class<S> serviceClass) {
        Retrofit retrofit = builder.client(httpClient).build();
        return retrofit.create(serviceClass);
    }
}

Previously I added in my gradle.build, I'm not sure if I should add GSON since they say Retrofit 1.9 has it but nothing is mentioned about Retrofit 2:

dependencies {  
    // Retrofit & OkHttp
    compile 'com.squareup.retrofit:retrofit:2.0.0-beta2'
}

EDIT

retrofit 2 is now stable. Use

compile 'com.squareup.retrofit2:retrofit:2.3.0'
compile 'com.squareup.retrofit2:converter-gson:2.3.0'

in your build.gradle dependency section

old answer

with Retrofit 2.0 you have to declare in your build.gradle the convert factory you want to use. Add

compile 'com.squareup.retrofit:converter-gson:2.0.0-beta2'

to your gradle and sync it again