Q:

How to parse RxJava to receive just the data that i want?

I just want the numbers, but this is what i'm receiving at log:

[Boletim(grade=4.5), Boletim(grade=9.5)]

The Response:
public class GradeResponse {

    @Inject
    Retrofit retrofit;

    @Inject
    MainPresenter mainPresenter;

    public void getGradeRx() {

        MyApplication.getMainComponent().injectIntoGradeResponse(this);// informando ao dagger sobre o uso de um component e a necessidade de injetar dependência

        Subscription getGrade = retrofit
                .create(GradeService.class)
                .getGrade()
                .subscribeOn(Schedulers.newThread())
                .observeOn(AndroidSchedulers.mainThread())
                .map(model -> {

                    return model.getBoletim();

                })
                .subscribe(new Observer<Boletim[]>() {
                    @Override
                    public void onCompleted() {

                    }

                    @Override
                    public void onError(Throwable e) {
                        Log.i(TAG, "saporra vai me matar ainda");
                    }

                    @Override
                    public void onNext(Boletim[] grades) {
                        Log.i(TAG, Arrays.asList(grades).toString());
                    }
                });
    }
}

The models:
GradeModel:
@SerializedName("boletim")
    @Expose
    private Boletim[] boletim;

Boletim.class
public class Boletim {

    @SerializedName("grade")
    @Expose
    private double grade;

The retrofit service is ok, the dependency injection is working. I'm receiving the onSuccess method from rxJava, i just need now receiving only the numbers without this "[Boletim(grade=".

A:

You are seeing the toString of your objects because you get the entire object in your map!

only the numbers without this "[Boletim(grade=".

Way can't you map again and extract it? 
.map(model -> {               // This map returns a Boletim[]
     return model.getBoletim();
 })
.map(boletim -> {             // This map returns a double[]
    Double grades = new Double[boletim.length];
    for (int i =0; i < grades.length ; i++) {
        grades[i] = boletim[i].getGrade() ;
    } 
    return grades;
}).subscribe(new Observer<Double[]>() {   // This subscribes to a double[]
    @Override
    public void onCompleted() {

    }

    @Override
    public void onError(Throwable e) {
        Log.i(TAG, "saporra vai me matar ainda");
    }

    @Override
    public void onNext(Double[] grades) {
        Log.i(TAG, Arrays.toString(grades));
    }
}

Or you could put the for loop into onNext 
If you don't want an Observable of arrays, then use flatMap 

