Here goes an exaple how to resize GIF-s.
<?php
$imagick = new Imagick("original.gif");
$format = $imagick->getImageFormat();
if ($format == 'GIF') {
  $imagick = $imagick->coalesceImages();
  do {
     $imagick->resizeImage(120, 120, Imagick::FILTER_BOX, 1);
  } while ($imagick->nextImage());
  $imagick = $imagick->deconstructImages();
  $imagick->writeImages('new_120x120.gif', true);
  $imagick = $imagick->coalesceImages();
  do {
     $imagick->resizeImage(100, 100, Imagick::FILTER_BOX, 1);
  } while ($imagick->nextImage());
  $imagick = $imagick->deconstructImages();
  $imagick->writeImages('new_100x100.gif', true);
}
$imagick->clear();
$imagick->destroy();
?>
You can use any resize method (resizeImage) with this example.
This example works fine with *imagick module version 2.2.1-dev*, but doesn't work correctly with *imagick module version 3.0.1*.