Over Kong Gun is not equal or even close to Dead Man Game level. Fans overrate the importance of knocking a character down because they misunderstood Zoro's comment. He wasn't talking about knocking Kaido off his feet. He was talking about making Kaido fall unconscious from his attack even if it was only temporarily.
Zoro saw Kaido get knocked down by Luffy's first attack and get right back up. The damage was similar to the damage he received from each Scabbards. Recreating that would be pointless. Even the Scabbards knocked Kaido over but couldn't we open his old wound.
Over Kong Gun knocked Kaido down but he immediately got back up. It didn't even slow him down.
While Dead Man Game severely injured or scarred Kaido, slow him down, caused his entire body to start shaking, and huffing. The first one to make Kaido huff.
Remember, it's Kaido Durban incredibly hard and durable skin that makes him hard to damage. It's easier to knock him over then to call him significant damage and that's why Zoro scarring Kaido was an incredibly amazing feat especially in Zoro's poor condition at the time.
