How to center GridView correctly?

You may want to put your GridView inside a RelativeLayout and then set GridView's layout_centerInParent property to true. something like this:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <GridView
            android:id="@+id/calendar_grid"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:numColumns="7"
            android:columnWidth="40dp"
            android:verticalSpacing="1dp"
            android:horizontalSpacing="1dp"
            android:paddingTop="2dp"
            android:stretchMode="columnWidth"
            android:background="#696969"
            android:layout_centerInParent="true" />

</RelativeLayout>

RelativeLayout gives you much better control over positions of children.

Here is my working configuration:

        <GridView
        android:id="@+id/gridView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:cacheColorHint="@color/white"
        android:verticalSpacing="10dp"
        android:horizontalSpacing="10dp"
        android:numColumns="3"
        android:padding="5dp"
        android:scrollbars="none"
        android:stretchMode="columnWidth"
        android:gravity="center"
        />