I create snippets for all my GTM Tags as sometimes we need to leave the GTM tag off.
I then add them using a Marketo variable to the global template in the head.
<div class="mktoSnippet" mktoName="Add Google Tag Manager Code" id="GTM"></div>
I also place a "No Script version just under the <Body> tag
<div class="mktoSnippet" mktoName="Add Google Tag Manager Code No Script" id="GTM_NoScript"></div>
It works for us but you need to remember to add the Snippet on each page build.
Clearer to use a boolean, no? (Unless you have multiple GTM "on" options.)