Note: The transformation was teased at the very end of Chapter 783.
Think about it:
Initially, the form's appearance was polarizing for fans and even laughed at by Doflamingo due to Luffy's spherical torso and constant bouncing. However, its effectiveness was undeniable. In this state, Luffy combines the speed of Gear 2 with the raw power of Gear 3, while his Haki-infused rubber skin becomes durable enough to bounce back Doflamingo’s attacks effortlessly. when do we first see gear 4