And Cracker blocked it with his Haki-fied shield, so they're even.Doflamingo completely blocked the attack with his Hakified arm so it reduced the damage a good bit, still hurt him though. He never took a direct blow to the chest like Cracker did, Cracker was totally off guard with no time to even use Haki or even use Hakified enforcement on his shield because he thought the battle was already won. Not to mention Luffy was literally right by him and he instantly turned G4 and simultaneously landed a blow on him so Cracker had 0 time to react.
That ain't no direct blow at all because if it was Mingo would have been hit in his chest, he wasn't. He still had his hands up after being knocked back by KG indicating it wasn't a direct hit.
Cracker was not off-guard. The page before shows that while sitting on top of Luffy, he had his shield off to the side of his body. Meaning that if Luffy managed to sneak attack him while he was off-guard with a punch, he wouldn't have been able to hit the shield. The fact that the shield was shattered shows that Cracker had to have moved the shield into the path of Kong Gun. The fact that the same page that shows the positioning of the shield to the side shows Cracker reacting to Gear 4th in shock means he had more than enough time to see Luffy coming, which is further proof that he was able to move his shield to mitigate some of the damage.
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If Luffy instantly hit him or whatever BS you're trying to say, we wouldn't get that final panel on that page of him reacting to Gear 4th with shock. The fact that Luffy is saying "Gomu Gomu no" while Cracker looks directly at him in what appears to be fear means he saw the KG coming.
All indications point to Cracker trying to defend himself and failing, just like Doflamingo.