Google Tag Manager is a FREE Magento 2 Extension that allows you to integrate Google Tag Manager to your site. This extension also provides ecommerce transaction tracking feature. You just need to enter your Tag Manager container id in the extension’s configuration settings.
This module is compatible with Magento version 2.0 and later.
The Magento 1.x version of this extension is present here.
Google Tag Manager allows you to quickly and easily update tags and code snippets on your website or mobile app, such as those intended for traffic analysis and marketing optimization. You can add and update AdWords, Google Analytics, Floodlight, and 3rd party or custom tags from the Google Tag Manager user interface instead of editing site code. This reduces errors, frees you from having to involve a web developer, and allows you to quickly deploy new features or content onto your site. More: https://support.google.com/tagmanager/answer/6102821?hl=en
This extension can be downloaded from Magento Marketplace:
Google Tag Manager (GTM)