activity：

public class MainActivity extends AppCompatActivity {
    private GridView gridView;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        gridView = findViewById(R.id.grid_view);
        gridView.setAdapter(new GridAdapter(this));
    }
}

layout：

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <GridView
        android:id="@+id/grid_view"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        android:layout_marginLeft="16dp"
        android:layout_marginRight="16dp"
        android:background="@color/colorPrimary"
        android:numColumns="3"

        />

</androidx.constraintlayout.widget.ConstraintLayout>

adapter

public class GridAdapter extends BaseAdapter {
    private LayoutInflater mInflater;
    public GridAdapter(Context context){
        mInflater = (LayoutInflater)context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    }
    @Override
    public int getCount() {
        return 9;
    }

    @Override
    public Object getItem(int position) {
        return null;
    }

    @Override
    public long getItemId(int position) {
        return 0;
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        convertView = mInflater.inflate(R.layout.item_grid,null);
        return convertView;
    }
}

item layout

<?xml version="1.0" encoding="utf-8"?>
<View xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="50dp"
    android:layout_height="50dp"
    android:background="@color/colorAccent"
    />

public Object getItem(int position) {
    return null;
}

你返回的都是null，没有数据

adapter 创建传入一个数据，在adapter中对item对应的布局进行赋值