AA using GTM?

I haven't come across any official documentation for doing this. Also, be aware that Client Care will not provide any support at all if you implement AA with GTM.

Having said that, this has worked for me before:

• In a Custom HTML tag that's fired with the All Pages trigger, add the AppMeasurement.js code.
• With other Custom HTML tags that are fired with appropriate triggers, add the s.t() and s.tl() calls as needed.

Just use the steps to deploy code using JavaScript implementation as described here: https://experienceleague.adobe.com/docs/analytics/implementation/js/overview.html?lang=en

You simply need to inject the AppMeasurement.js library and then instantiate Adobe Analytics. Also you will need to deploy Visitor ID services and instantiate it before Adobe Analytics.

These steps are if you do not use AEP.