Yeah, not traditional QBs. Guys who either have the arm talent and the utter disregard for the play call when it doesn't make sense with what the defense presents, like Mahomes, Allen, Burrow, Stafford and previously Brady and Rodgers (he's cooked now) or the leg talent to lay waste to the other team's defensive plans, so, Hurts and Jackson (also maybe cooked???) and then also Allen again. Maye, Daniels and (hopefully not) Caleb Williams may very well be on the verge of being gunslingers too.
I know the term is still fairly vague and kind of an I'll-know-it-when-I-see-it situation, but I think there is a really good test for this. Take the final play of the 2022 Wild Card game against the Giants. The 'gunslinger' QBs would have either heard the utterly terrible play call from the coach and changed it on the spot or run the play, scrambling out of the pocket directly at the LB forcing him into the no-win decision of trying to tackle the QB or defending Hockenson from getting the 1st down.
Not trying to rip Cousins there, I'm just trying to explain how I view these other QBs differently.