I get it. But as that would mean the FA would have to deal with him at the end of the flight when he is pissed off that his bag is missing, then later learns it was gate checked without his knowledge - I would rather an FA invite that on themselves if they wish. So my default action would be to have the passenger deal with it so if it did get gate checked, he wouldn't take it out on them at the end of the flight. Personally, I choose when to confront someone (and often only after there is no other recourse) but I try not to place that burden on someone else without their permission.