Wow TIL. I thought what we call gartel chabad calls bendel. Wearing a gartel under the shirt is a total chidush for me.
How about wearing a gartel on top of a shirt (with no jacket)?
For some reason it seems to me that it is something that only those that have some sort of affiliation with Chabad do (even though IMHO one who considers himself a Lubavitcher should not be davening like this, but should wear a jacket during davening.
Also, the function of the bendel as a shabbos key bendel cannot be understated.