The Fourth Gear: Gomu Gomu no Mi

While Luffy is using Gear 4, his physical strength and speed are increased to the point where he is able to overwhelm a Warlord of the Sea, Donquixote Doflamingo, and the second strongest member of an Emperor's crew, Charlotte Katakuri, whereas before, Gear 2's lack of strength and Gear 3's lack of speed made them ineffective in the fight.