>>280922411
Rocks never died. Roger and WG decided to turn the massacre game into a Davey Back fight, they actually all played for the treasure boxes. Garp was the deciding victory in a boxing match just like Luffy and got declared hero of the marines for requesting their flag forcing them to disband, hence the reason we never actually saw the rest of the God Valley flashback.

What really happened is Big Mom won the eating contest and they let her keep the fish fish fruit and she gave it to Kaido.

Source: Trust me