There is quite a few documentation about how to implement Anaytlics eCommerce tracking on your site, but not enough on how to accompish it thru Drupal 8. The following steps will concentrate mainly on drupal's side of this setup considering you already followed Google instructions on how to setup GTM and connect it with GA.

- First step is to add Google provided GTM script to the head of your site (See Google implemenation tutorial)

- Setup GTM to pass on data thru to Google Analytics

- From the root of your site access and edit `/themes/custom/[your-theme]/templates/commerce/commerce-checkout-completion-message.html.twig`

- Add the following code anywhere in this file

{% set shippingAmount = 0.00 %}

{% for adjustment in order_entity.getAdjustments %}
    {% if(adjustment.getType == "shipping") %}
        {% set shippingAmount = adjustment.amount.getNumber %}
    {% endif %}
{% endfor %}
<script>
    dataLayer.push({
        'transactionId': '{{ '@number'|t({'@number': order_entity.getOrderNumber}) }}',
        'transactionAffiliation': 'YourSite.com',
        'transactionTotal': {{ order_entity.getTotalPrice | striptags | trim | replace({'USD' : ''})  }},
        'transactionShipping': {{ shippingAmount | number_format(2, '.', ',')}},
        'transactionProducts': [
            {% for order_item in order_entity.getItems %}
            {
                'quantity': {{ order_item.getQuantity|number_format }},
                'name': '{{ order_item.label }}',
                'sku': '{{ order_item.getPurchasedEntity.getSku }}',
                'price': {{ order_item.getTotalPrice|commerce_price_format|striptags|trim|replace({'$' : ''}) }}
            },
            {% endfor %}
        ]
    });
</script>