Luffy sure amazed us and looked so “hardo boildo” when he showed the G2 for the first time, it was a new power-up and it was the first time he showed how useful his Devil Fruit actually is. Right after that he exceeded all out expectations by showing G3. It was the best fight ever! Half giant Luffy and with the side affect (the side affect is that Luffy becomes small after it’s use) was going way over the top.
Now on the current Dressrosa arc, we have seen the new power-up – G4(still only in manga version). It kinda looks like Nightmare Luffy.
Luffy coats his arm in Busoshoku Haki before biting into his forearm. Similar to how he activates Gear Third, he blows an incredible amount of air into his body, but this time he inflates his muscular structure before distributing the air throughout his body, with emphasis on his upper half. Luffy’s proportions become warped and his body becomes much larger in size, with his arms, upper torso, and legs coated in Busoshoku Haki. The coating across his torso resemble the wispy ends made from flames, giving it the appearance of tattoos, and he also gains shading around the inner and outer edges of his eyes. He also physically grows taller, standing around 4 meters tall.
Now will there even be a Gear 5? What could G5 be? Could it perhaps have something to do with Smoker? Jokes aside, I can’t wait to see that one day.