SalesEventQuoteMerge.php

<?php
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
declare(strict_types=1);

namespace Magento\GiftMessage\Observer;

use Magento\Framework\Event\ObserverInterface;
use Magento\Quote\Model\Quote;

/**
 * Gift Message Observer Model
 */
class SalesEventQuoteMerge implements ObserverInterface
{
    /**
     * Sets gift message to customer quote from guest quote.
     *
     * @param \Magento\Framework\Event\Observer $observer
     * @return $this
     */
    public function execute(\Magento\Framework\Event\Observer $observer)
    {
        /** @var  Quote $targetQuote */
        $targetQuote = $observer->getData('quote');
        /** @var  Quote $sourceQuote */
        $sourceQuote = $observer->getData('source');

        $giftMessageId = $sourceQuote->getGiftMessageId();
        if ($giftMessageId) {
            $targetQuote->setGiftMessageId($giftMessageId);
        }

        return $this;
    }
}