Magento – get Qty Increments

Peter, you're trying to use wrong functionality. Quantity increments are meant to be used in cases where you absolutely have to sell something that can't be sold separately, such as six beers in a special package, as doing otherwise decreases its value. This means that you can only choose following quantities: 6, 12, 18, 24 and so on. What YOU want to do is to use bundles: create a new Bundle Product, add associated products as REQUIRED check-box (quantity 100). This way you can choose put one bundle to shopping cart while decreasing stock status of original product by 100 (or in my case by 6). If you need more information search the internet for how to create bundled products, there're TONS of excellent tutorials for scores of languages.

Try following way:

public function __construct(
    \Magento\Sales\Api\OrderRepositoryInterface $orderRepository,
    \Magento\CatalogInventory\Model\Stock\StockItemRepository $stockItemRepository
) {
    $this->orderRepository = $orderRepository;
    $this->stockItemRepository =$stockItemRepository;
}

And your calculation should be:

$order = $this->orderRepository->get(13);
if($order->getEntityId()) {
    $items = $order->getItems();
    foreach ($items as $item) {
        $stock = $this->stockItemRepository->get($item->getProductId());
        $remainingShipQty = $stock->getQty()-$item->getQtyShipped();
    }
}

[Update]

You need to log data into file. Echo not working for observer.

namespace Custom\Module\Observer;

use Magento\Framework\Event\Observer;
use Magento\Framework\Event\ObserverInterface;
use Magento\Framework\App\ObjectManager;

class OrderPlaceAfter implements ObserverInterface 
{
    /** @var \Magento\Framework\Logger\Monolog */
    protected $logger;
    protected $resources;
    protected $orderRepository;
    protected $stockItemRepository;

    public function __construct(
        \Psr\Log\LoggerInterface $logger,
        \Magento\Framework\App\ResourceConnection $resources,
        \Magento\Sales\Api\OrderRepositoryInterface $orderRepository,
        \Magento\CatalogInventory\Model\Stock\StockItemRepository $stockItemRepository
    ) {
        $this->logger = $logger;
        $this->resources = $resources;
        $this->orderRepository = $orderRepository;
        $this->stockItemRepository = $stockItemRepository;
    }

    public function execute(Observer $observer)
    {

        $orderId = 110; // My Order Number

        $order = $this->orderRepository->get($orderId);

        if ($order->getEntityId()) {
            $items = $order->getItems();
            foreach ($items as $item) {
                $stock = $this->stockItemRepository->get($item->getProductId());
                //echo $stock->getQty().'-'.$item->getQtyShipped();
                $remainingShipQty = $stock->getQty() - $item->getQtyShipped();
                $this->logger->info($remainingShipQty);
            }
        }
        return $this;
    }
}