After see many frontend developers search for how call gallery in product list page , we decide to write small course (tutorial) for how done this task .

Ok first you need create class helper inside your module , for example Ibnab/Common/Helper/Data.php and push the code :

 
<?php
/**
 * Copyright © 2015 Magento. All rights reserved.
 * See COPYING.txt for license details.
 */
namespace Ibnab\Common\Helper;
use Magento\Catalog\Model\Product\Gallery\ReadHandler as GalleryReadHandler;
class Data extends \Magento\Framework\App\Helper\AbstractHelper {
  protected $galleryReadHandler;
    /**
     * Catalog Image Helper
     *
     * @var \Magento\Catalog\Helper\Image
     */
    protected $imageHelper;
    public function __construct(
    GalleryReadHandler $galleryReadHandler,  \Magento\Framework\App\Helper\Context $context,\Magento\Catalog\Helper\Image $imageHelper)
    {
        $this->imageHelper = $imageHelper;
        $this->galleryReadHandler = $galleryReadHandler;
        parent::__construct($context);
    }
   /** Add image gallery to $product */
    public function addGallery($product) {
        $this->galleryReadHandler->execute($product);
    }
    public function getGalleryImages(\Magento\Catalog\Api\Data\ProductInterface $product)
    {
        $images = $product->getMediaGalleryImages();
        if ($images instanceof \Magento\Framework\Data\Collection) {
            foreach ($images as $image) {
                /** @var $image \Magento\Catalog\Model\Product\Image */
                $image->setData(
                    'small_image_url',
                    $this->imageHelper->init($product, 'product_page_image_small')
                        ->setImageFile($image->getFile())
                        ->getUrl()
                );
                $image->setData(
                    'medium_image_url',
                    $this->imageHelper->init($product, 'product_page_image_medium')
                        ->constrainOnly(true)->keepAspectRatio(true)->keepFrame(false)
                        ->setImageFile($image->getFile())
                        ->getUrl()
                );
                $image->setData(
                    'large_image_url',
                    $this->imageHelper->init($product, 'product_page_image_large')
                        ->constrainOnly(true)->keepAspectRatio(true)->keepFrame(false)
                        ->setImageFile($image->getFile())
                        ->getUrl()
                );
            }
        }
        return $images;
    }
}
 

Here we call to objects  Magento\Catalog\Model\Product\Gallery\ReadHandler and \Magento\Catalog\Helper\Image from it's name you can observe the first is the reader of gallery by attribute - here thery reads media_gallery - and giving you as result information of all images of this current product passed to method $this->galleryReadHandler->execute($product)   .
So and \Magento\Catalog\Helper\Image in our example is to organize our gallery to three type  image_small  / image_medium / image_large .

Ok now you need go to your list.phtml inside your theme or anywhere and calling your helper :

$_helperGallery = $this->helper('Ibnab\Common\Helper\Data');

Now you can use with the current product of foreach statement (with your technique):

 
                   <a href="<?php echo $_product->getProductUrl() ?>">
                            <ul class="product-item-wrapper">
                                <?php
                                $_helperGallery->addGallery($_product);
                                $images = $_helperGallery->getGalleryImages($_product);
                                if ($images instanceof \Magento\Framework\Data\Collection) {
                                    $i = 1;
                                    foreach ($images as $image) {
                                        $item = $image->getData();
                                        if (isset($item['media_type']) && $item['media_type'] == 'image'):
                                            ?>
                                            <?php if ($i == 1): ?>
                                                <li class="selected">
                                                <?php else: ?>
                                                <li >
                                                <?php endif; ?>
                                                <img src="<?php echo isset($item['medium_image_url']) ? $item['medium_image_url'] : null; ?>" alt="Preview image">
                                            </li>
                                            <?php
                                            $i++;
                                        endif;
                                    }
                                }
                                ?>
                            </ul>
                        </a>
 

You can  dump current image inside loop statement with :
var_dump( $image->getData());

now all is cool you can use any type of image from gallery