His gear 2 is based on an elastic band. The gear three is based on on the concept of a balloon. The gear 4 is based on bouncy ball but is elastic as well. So, what will the gear 5 look like? Remember what Doflamingo said about Luffy? He said even though he’s coated in armament Haki he’s still made of rubber.
So, will his body become some sort of hard rubber or should I say Vulcanised rubber? Vulcanised rubber is a form of rubber that is obtained by a chemical process and is far more durable than the normal rubber. When someone hits you with an elastic band you feel a bit of pain. Now, if Luffy hits someone with his elastic rubber form or gear 2, it has the power to defeat a giant in one shot. But an elastic band is a soft type of rubber and with it, Luffy defeated a giant. But that’s not all rubber is capable of. A stronger version of rubber would obviously hurt a lot more, and will have a lot more durability as well.