I've noticed the Google Tag Manager tag in the Tag Marketplace for a couple months now. It appeared with little fanfare. I have a use case for it coming soon, and I'd like to learn more about it from Tealium employees and fellow TiQ users....
So far I've browsed the template and it seems rather flexible/straightforward, but I'd love to see additional documentation and/or discussion.
In my experience, we have used the GTM tag to assist customers who are migrating from GTM to TiQ, either so that they can migrate tags from GTM to TiQ and then easily remove the GTM container when they are ready.
Alternatively some customers that have specific reasons that mean they 'must' retain GTM on some sub-domains or for certain inbuilt hard-coded tracking events, but they still want global control of all of their tags from TiQ.
It's not a tag that you would normally need to use unless you have a very specific situation, so it might be worth checking with your CSM or the CSE team before implementing it
That said, it's actually super simple to add, just add the tag and fill in the details (see screenshot) - the key is lots of testing and making sure it doesn't conflict with a native GTM container tag on the site, other than that there are really just two things to fill in.