Selected answers from the Dumb SEO Questions G+ community.
Benj Arriola: Never thought of doing that but if it is the JSON-LD format, I can see how it will be easy to implement on a few pages without touching the code of the site.
However if you want to implement this for a very large site, with hundred thousand to millions of pages, that would be very difficult to implement via GTM alone. I suggest working on it server side scripting instead, integrating it with the CMS database to see how all of the structured data be implemented in all pages using backend code.
Abindra Raj Dangol: Thanks for your help. I was just wondering if it has negative impact even for the few pages.
Richard Hearne: Bear in mind that anything that requires JS to render into page will take longer to be extracted and indexed on Google`s side, and is less likely to be updated in a timely way. If your strucutured data is set-it and forget-it then this should be OK, but if it is dynamic you might be best off rendering to the served HTML.