The extra attack you gain from the great weapon master triggers when you kill something. While critting does a lot of damage, it's not a guaranteed kill. Also you have to use your bonus action to do the attack. And you only get one bonus action per turn
So at level five. You can make two attacks every turn, guaranteed. If you kill something then you can use your bonus action to take another attack, for a total of three. Each of those crit on a 19 or 20, cuz champion