gtag (global site tag) and Tag Manager are two Google products that can help you track the traffic to your websites or mobile apps.
Key difference:
<ul>
 <li>gtag: aggregate all tags for google products (ads, analytics)</li>
 <li>Tag Manager: not limited to Google products, can also include Facebook Pixel or any javascript based tool that has a tracking code you can install.</li>
</ul>
Tag Manager is much more powerful; gtag is essentially a mini versin of Tag Manager, good for beginners.

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