Right now this function doesn't appear to do anything, ref: http://stackoverflow.com/q/10739822/2685496
As mentioned in the answer, modulateImage works fine as a replacement for converting to gray scale.
<?php
$image = new Imagick("input.jpg");
$image->modulateImage(100, 0, 100);
$image->writeImage("output.jpg");
$image->clear();
?>