Such Forms are based on how much does these Characters push their Bodies,
Luffy have Gear 2 (Attack Speed) ---> Zoro have Gentle Blade
Luffy have Gear 3 (Attack Size/Weight) ---> Zoro have Strong Blade
Luffy have Gear 4 (Temporarily Fight at 100%) ---> Zoro have King of Hell
Luffy have Gear 5 (Mythical Form beyond his Limit) ---> Zoro have Asura (Not an Illusion)
The only reason Luffy's are called Transformations or Forms is because He Fights with his Body, so his Upgrades would be visible on his Body obviously lol while Zoro Fights with Swords, so his Upgrades depend on how much He feeds them, and his Mythical Form multiplies his Swords.