Unable to create converter for my class in Android Retrofit library

I'm migrating from using Volley to Retrofit, I already have gson class that I used before for converting JSONObject response to a object that implements gson annotations. When I'm trying to make a HTTP GET request using Retrofit but then my app crashes with this error:

 Unable to start activity ComponentInfo{com.lightbulb.pawesome/com.example.sample.retrofit.SampleActivity}: java.lang.IllegalArgumentException: Unable to create converter for class com.lightbulb.pawesome.model.Pet
    for method GitHubService.getResponse

Im following the guide in retrofit site and I come up with these implementations:

This is my activity where I am trying to execute the retro http request:

public class SampleActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_sample);

        Retrofit retrofit = new Retrofit.Builder()
                .baseUrl("**sample base url here**")
                .build();

        GitHubService service = retrofit.create(GitHubService.class);
        Call<Pet> callPet = service.getResponse("41", "40");
        callPet.enqueue(new Callback<Pet>() {
            @Override
            public void onResponse(Response<Pet> response) {
                Log.i("Response", response.toString());
            }

            @Override
            public void onFailure(Throwable t) {
                Log.i("Failure", t.toString());
            }
        });
        try{
            callPet.execute();
        } catch (IOException e){
            e.printStackTrace();
        }

    }
}

My interface which turned to be my API

public interface GitHubService {
    @GET("/ **sample here** /{petId}/{otherPet}")
    Call<Pet> getResponse(@Path("petId") String userId, @Path("otherPet") String otherPet);
}

And finally the Pet class which should be the response:

public class Pet implements Parcelable {

    public static final String ACTIVE = "1";
    public static final String NOT_ACTIVE = "0";

    @SerializedName("is_active")
    @Expose
    private String isActive;
    @SerializedName("pet_id")
    @Expose
    private String petId;
    @Expose
    private String name;
    @Expose
    private String gender;
    @Expose
    private String age;
    @Expose
    private String breed;
    @SerializedName("profile_picture")
    @Expose
    private String profilePicture;
    @SerializedName("confirmation_status")
    @Expose
    private String confirmationStatus;

    /**
     *
     * @return
     * The confirmationStatus
     */
    public String getConfirmationStatus() {
        return confirmationStatus;
    }

    /**
     *
     * @param confirmationStatus
     * The confirmation_status
     */
    public void setConfirmationStatus(String confirmationStatus) {
        this.confirmationStatus = confirmationStatus;
    }

    /**
     *
     * @return
     * The isActive
     */
    public String getIsActive() {
        return isActive;
    }

    /**
     *
     * @param isActive
     * The is_active
     */
    public void setIsActive(String isActive) {
        this.isActive = isActive;
    }

    /**
     *
     * @return
     * The petId
     */
    public String getPetId() {
        return petId;
    }

    /**
     *
     * @param petId
     * The pet_id
     */
    public void setPetId(String petId) {
        this.petId = petId;
    }

    /**
     *
     * @return
     * The name
     */
    public String getName() {
        return name;
    }

    /**
     *
     * @param name
     * The name
     */
    public void setName(String name) {
        this.name = name;
    }

    /**
     *
     * @return
     * The gender
     */
    public String getGender() {
        return gender;
    }

    /**
     *
     * @param gender
     * The gender
     */
    public void setGender(String gender) {
        this.gender = gender;
    }

    /**
     *
     * @return
     * The age
     */
    public String getAge() {
        return age;
    }

    /**
     *
     * @param age
     * The age
     */
    public void setAge(String age) {
        this.age = age;
    }

    /**
     *
     * @return
     * The breed
     */
    public String getBreed() {
        return breed;
    }

    /**
     *
     * @param breed
     * The breed
     */
    public void setBreed(String breed) {
        this.breed = breed;
    }

    /**
     *
     * @return
     * The profilePicture
     */
    public String getProfilePicture() {
        return profilePicture;
    }

    /**
     *
     * @param profilePicture
     * The profile_picture
     */
    public void setProfilePicture(String profilePicture) {
        this.profilePicture = profilePicture;
    }


    protected Pet(Parcel in) {
        isActive = in.readString();
        petId = in.readString();
        name = in.readString();
        gender = in.readString();
        age = in.readString();
        breed = in.readString();
        profilePicture = in.readString();
    }

    @Override
    public int describeContents() {
        return 0;
    }

    @Override
    public void writeToParcel(Parcel dest, int flags) {
        dest.writeString(isActive);
        dest.writeString(petId);
        dest.writeString(name);
        dest.writeString(gender);
        dest.writeString(age);
        dest.writeString(breed);
        dest.writeString(profilePicture);
    }

    @SuppressWarnings("unused")
    public static final Parcelable.Creator<Pet> CREATOR = new Parcelable.Creator<Pet>() {
        @Override
        public Pet createFromParcel(Parcel in) {
            return new Pet(in);
        }

        @Override
        public Pet[] newArray(int size) {
            return new Pet[size];
        }
    };
}

Solution

Prior to 2.0.0, the default converter was a gson converter, but in 2.0.0 and later the default converter is ResponseBody. From the docs:

By default, Retrofit can only deserialize HTTP bodies into OkHttp's ResponseBody type and it can only accept its RequestBody type for @Body.

In 2.0.0+, you need to explicitly specify you want a Gson converter:

Retrofit retrofit = new Retrofit.Builder()
    .baseUrl("**sample base url here**")
    .addConverterFactory(GsonConverterFactory.create())
    .build();

You will also need to add the following dependency to your gradle file:

compile 'com.squareup.retrofit2:converter-gson:2.1.0'

Use the same version for the converter as you do for your retrofit. The above matches this retrofit dependency:

compile ('com.squareup.retrofit2:retrofit:2.1.0')

Also, note as of writing this, the retrofit docs are not completely updated, which is why that example got you into trouble. From the docs:

Note: This site is still in the process of being expanded for the new 2.0 APIs.