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You do not need to initialise GA with code, you should be doing this within the GTM GUI.
I expect the product detail push is with the pageview which GA tracks by default when the DOM loads.
Once you set up GA using GTM you’ll need to pass the event to GA on the trigger which is the dataLayer.push event name. To do this create a custom event trigger in GTM and then send the data in the payload to GA.
You’ll also need to configure the GA tracking in GTM to use the dataLayer for your e-commerce data.
The GTM preview will show you what is and isn’t being passed to the dataLayer, you need to create triggers that fire from the dataLayer events.
Source:stackexchange.com