This code is available on pre API21 with support library!
I have a simple piece of code to show 4 buttons in a gridLayout of 2 columns that take 50% of the available space: perhaps it can help
<GridLayout
android:id="@+id/grid"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:columnCount="2"
>
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Button"
android:layout_gravity="fill"
android:layout_columnWeight="1"
/>
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Button"
android:layout_gravity="fill"
android:layout_columnWeight="1"
/>
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Button"
android:layout_gravity="fill"
android:layout_columnWeight="1"
/>
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Button"
android:layout_gravity="fill"
android:layout_columnWeight="1"
/>
</GridLayout>
Solution is perhaps this :
android:layout_gravity="fill"
android:layout_columnWeight="1"