I have worked with COS to implement Google Tag Manager on esp.gmu.edu.
Previously they were using regular Google Analytics via a WordPress plugin.
I have tested and confirmed that esp.gmu.edu is now implementing an instance of the Mason Standard Google Tag Manager container, and it is correctly sending tracking info to its own website analytics property, as well as the new COS roll-up property (which will aggregate all COS websites), as well as to the university roll-up property (which aggregates all university websites).