Luffy used kong gun last chapter is that his ultimate attack? Gamma knife isn’t his ultimate attack. It’s a strong attack but saying it’s his strongest is dumb.
As far as we know, Luffy's ultimate attack is NOT kong gun
As far as we know too, Law's trump card against Mingo was his Gamma Knife
Now, there could be something else up his sleeves, BUT:
1- We know that during his Mingo fight, he pretended to be dead JUST to deliver that attack, as his biggest wild card
2- Last chapter, this was his plan, he planned to effect Kaido from the inside by using Gamma Knife...
So.... again, Oda can write some new move for him just to pander for Law as supernova and not leave him in the dust.. BUT from the intel we knew, Gamma knife was his BEST choice for this war to do any damage... which is why I'm saying that it's so underwhelming that when he used the move, nothing in spoiler referred to any serious damage....
Hope my point is clear