background_gradient.xml

// res/drawable
<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android" android:shape="rectangle">
    <gradient
        android:startColor="@android:color/black"
        android:centerColor="@color/black_overlay"
        android:endColor="@android:color/transparent"
              
        android:angle="90"
        />
</shape>


// res/color
<color name="black_overlay">#99000000</color>