I’ve seen a lot of people wondering, and I wondered as well; why is Luffy’s Zoan fruit so different from the other Zoan fruits? The answer, I believe, is that it isn’t.
Luffy’s fruit’s behavior prior to awakening is completely in line with one specific line of Zoan fruits, which it itself is a part of: the Mythical Zoan class.
The Mythical Zoans are obviously a bit different from regular Zoans as they have additional powers, Marco’s blue flames, Sengoku’s shockwaves. The explanation given in this latest chapter is that Luffy’s rubber body is a result of that power.
However, Mythical Zoans also have one other distinct advantage over the other regular Zoan fruits: Mythical Zoan users can use their abilities while in their base, human form, which I believe is what Luffy has been doing up until now.
Just think about it. Marco can use his blue flames and only transform certain parts of his body, something other Zoans can’t do. Kaido is able to continue holding up Onigashima with the flame clouds from his dragon form in his base form. Yamato is able to use ice attacks with her Kanabo in her base form. Katarina Devon is able to transform in her base form. Sengoku hasn’t shown it, but I bet he’s able to use shockwaves in his base form as well.
This unique power of Mythical Zoans is an explanation for why Luffy is able to be rubber and use rubber abilities in his Zoan base form. Forms like gear second, and gear third could be seen as Luffy’s hybrid Nika form, and gear fourth can be seen as Luffy’s Nika transformation. Hyogoro called Luffy a guardian deity in Gear 4, so that’s why I believe that it is his full transformation.
Luffy’s awakening allows him full access to all of Nika’s abilities at once, as well as enhanced strength and speed, as well as environmental manipulation.
TLDR: Mythical Zoans are the only type of Zoans to be able to use their special abilities in base form, which explains how Luffy is rubber in base form. Luffy’s second and third gears are his hybrid forms, while gear fourth is his full transformation.
Theory by slshillcutt (https://www.reddit.com/user/slshillcutt)