Can I make the color of a toolbar a gradient?

Programmatically you can set Background of toolbar like this

toolbar.setBackgroundResource(R.drawable.gradient_one);

Your gradient may look like this

<shape
  xmlns:android="http://schemas.android.com/apk/res/android"
  android:shape="rectangle"
>
  <corners android:radius="9dp"/>

  <gradient
    android:startColor="#cc4151"
    android:centerColor="#d95a5e"
    android:endColor="#e27e70">
  </gradient>
</shape>

You can also set the gradient color programmatically through the ActionBar

ActionBar actionBar = getSupportActionBar();
actionBar.setBackgroundDrawable(AppCompatResources.getDrawable(context, R.drawable.toolbar_background));

toolbar_background drawable with the gradient colors as follows:

<shape xmlns:android="http://schemas.android.com/apk/res/android"
android:shape="rectangle">
    <gradient
        android:angle:="45"
        android:startColor="#F3A183"
        android:endColor="#EC6F66">
    </gradient>
    <size android:width="24dp"/>
    <size android:height="24dp"/>
</shape>