<!-- Here centerX does not work -->
<shape
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:shape="rectangle">

    <gradient
        android:centerX="30%"
        android:startColor="@android:color/transparent"
        android:endColor="@android:color/white"/>

</shape>

<!-- Here the startColor turns greyish -->
<shape
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:shape="rectangle">

    <gradient
        android:centerColor="@android:color/white"
        android:centerX="30%"
        android:endColor="@android:color/white"
        android:startColor="@android:color/transparent" />

</shape>

<?xml version="1.0" encoding="utf-8"?>
<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:left="100dp">
        <color android:color="@android:color/white"/>
    </item>
    <item android:width="100dp">
        <shape
            xmlns:android="http://schemas.android.com/apk/res/android"
            android:shape="rectangle">
        <gradient
            android:centerX="30%"
            android:endColor="@android:color/white"
            android:startColor="@android:color/transparent"/>
        </shape>
    </item>
</layer-list>

<shape
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:shape="rectangle">

    <gradient
        android:startColor="#00ffffff"
        android:centerColor="#00ffffff"
        android:endColor="#ffffffff"
        android:centerX="30%" />
</shape>