With PECL Imagick extension and using PHP

resize_animated_gif.php

<?php

$maxWidth = 200;
$maxHeight = 200;

try {
	$imagick = new Imagick();

	$fh = fopen(__DIR__ . '/batman.gif', 'r');
	$imagick->readImageFile($fh);

	$newImagick = $imagick->coalesceImages();

	$width  = $newImagick->getImageWidth();
	$height = $newImagick->getImageHeight();

	$fitByWidth = (($maxWidth / $width) < ($maxHeight / $height)) ? true : false;

	$maxWidth  = ($fitByWidth) ? $maxWidth : 0;
	$maxHeight = ($fitByWidth) ? 0 : $maxHeight;

	foreach ($newImagick as $k => $frame) {
	    $newImagick->thumbnailImage($maxWidth, $maxHeight, false);
	}

	header('Content-Type: image/' . $newImagick->getImageFormat());

	echo $newImagick->getImagesBlob();
} catch (Exception $e) {
	die($e->getMessage());
}