So first, Latin Rite laypeople copied the gesture on their own initiative.
Then the GIRM tried to decree that every layperson must do it, but nobody bothered to actually tell laypeople. And it’s still optional in the EF and the Anglican Use.
Heh, heh, typical bureaucratic silliness. There are some things that nobody should attempt to legislate. In this case, you see the authorities going against custom by trying to legislate custom… and so they get ignored by custom.
One of the ancient rules in all the Rites is that laypeople don’t really have rubrics for their worship, or only the most minimal ones. That’s why different areas have the laypeople doing slightly different things… and that’s perfectly okay! Laypeople are not clerics, and our spiritual duties and needs at Mass are different. The folklore of what Catholic laypeople do at Mass, their silent add-on prayers, etc. is generally very edifying and very personal. That personal nature is what makes it special and meaningful; if it were demanded, it would be burdensome.
On the other hand, you also see that “everybody knows” what prayer goes along with the gesture, but the cleric reading the Gospel does not actually have that prayer in the rubrics for him to say! It’s a sotto voce prayer entirely created and maintained by custom! (And in this case, the GIRM does not attempt to legislate it. Which is good.) But since historically clerics are much more tightly regulated in their conduct at Mass, it’s amusing to see them getting the freedom to act while laypeople get bound.
So there’s an interesting contrast of legal approaches within the space of a paragraph in the GIRM, and with regard to the same gesture. Bureaucratic silliness, as I said. You probably won’t see it in the next edition.