Even for a full widebody these days I rarely see more than one GA dedicated to boarding. When there is more than on GA, it seems to be the airlines' decision to provide a higher level of touch, not a need. One GA can board people faster than people can settle in on the plane.

Even if everyone were using the facial recognition boarding, they would still need to have a GA monitoring the process - helping people with problems, watching bags, etc. So it's not clear to me how this leads to any efficiency.