King of Hell is basically Zoro's Gear 4
It's just that Luffy Unlocked Gear 4 & then CoC Coating, while Zoro Unlocked them together simultaneously
On ther hand, Zoro had Asura before Luffy had Gear 5
Luffy had Kong Gun for example which was a Gear 4 Move, after Upgrading that Move with CoC, He named it Over Kong Gun.
This is why during King of Hell, Zoro has "King of Hell Three-Sword Style", and then the Stronger Version "Dragon Serpent"
Basically, Luffy & Zoro have Identical Arsenal:
Gear 1 - Two-Sword Style
Gear 2 - One-Sword Style
Gear 3 - Three-Sword Style
Gear 4 - King of Hell Style
- Gear 4 Boundman (Max Strength) - King of Hell Three-Sword Style (Max Strength)
- Gear 4 Snakeman (Max Speed) - King of Hell One-Sword Style (Max Speed)
Gear 5 - Asura (Nine-Sword Style)
- We don't know yet if They gonna have Variations as well
And each Fighting Style has a CoA-Enhanced Variation + CoC-Enhanced Variation
Except Gear 4 & King of Hell who already are CoA-Based Forms, so They can't be Used without CoA
Oda can't draw every single Variation of every single Style, We just need to know that They exist