As far as I’m aware, the Google Analytics tracking code needs to be added just after the <body> tag. In Moxie, the <body> tag is output in the ‘header.php’ template.
Keep in mind that if you modify core theme files then each time you update the theme you will need to make all of those modifications again to the updated theme files. If you are going to customise PHP templates it’s best to do that in a child theme because that will make it easier for you to update the parent theme. There are more details about how to create a child theme for Moxie here → http://para.llel.us/support/tutorials/making-a-moxie-child-theme/
I’ve never used the Google Tag Manager plugin so I can’t really help much with it. If you need assistance with any third-party plugin, please ask the plugin author for support.
Thanks.
