The God Valley incident is suspected to happened on the very same day on which celestial dragons were conducting a native island hunting game. In this game, Celestial Dragons wipe out the unaffiliated island and then surrender it to the World Government.
During this event, the slaves decides too get hand over a devil fruit and save others. Kuma manages to find and ingest Paw-Paw Devil Fruit.
On getting the intel of Roger, The Hero of Marines enters the God Valley. However, it is the later development that he allies with Roger Pirates to defeat Rocks Pirates.
it is obviously clear that the Rocks Pirates were at their prime, with fearsome pirates in it. It took the combined efforts of Garp and Roger to defeat Rocks Pirates. After this confrontation, Garp was monikered as The Hero of Marines.
The most fearsome and notorious pirates of the new era, Rocks Pirates, were defeated for the very first time.
One Piece Chapter 1096 revealed a number of tweaks, however, the legendary battle and its gossips are yet to air in reality.