Hi,
We use GTM and Munchkin together. Create a new tag in GTM with the type "Custom HTML Tag". Copy and paste your Munchkin code into the HTML field. Then, set a rule to fire it. You probably want to fire it everywhere on your website, in which case you would use URL matches regex .* (copy and paste the ".*" into the blank) For us, we blocked out our documentation section, as it gets a ton of unrelated traffic.
Cheers,