Hi @Jeff !
There is a way to include something like GTM within a Builder page using a Custom Code block, as shown here: Is there a good way to include Facebook Pixel or Google Analytics Events in Builder?
But I actually think it sounds like in this case the best way would be to set up GTM within your app itself. With 3rd party analytics trackers like GTM, Builder is totally agnostic, which is to say that as long as you have it set up in your app then Builder shouldn’t really interfere or interact with GTM. For setting up Tag Manager, I would recommend looking at Google’s docs: Setup and install Tag Manager - Tag Manager Help
As they will most likely be able to best assist you. Let me know if that answers your question or if we can help further!