The real name of the Gomu Gomu No Mi (rubber fruit) is Nika Nika No Mi (grin/smile fruit).
Sometimes Oda likes to use numbers, here’s a quote from SBS Vol.59:
Question: Odacchi! I noticed an amazing thing!!! It is that if you show the Straw Hat Devil Fruit users in numbers, it will be the numbers from 1~10!!
Luffy→Gomu Gomu (184.108.40.206.), Chopper→Hito Hito (220.127.116.11), Robin→Hana Hana (18.104.22.168.), Brook→Yomi Yomi (22.214.171.124.)
(126.96.36.199.188.8.131.52), but there is no (2.9), so is the one who ate the Nikyu Nikyu (184.108.40.206.) no Mi, Bartholomew Kuma, gonna be a Straw Hat next? Please tell me!!! P.N. SHANKUS.LOVE
Oda: WHAAAAAT?! (shock) Gosh that surprised me! Why was I surprised?! NO COMMENTTTTTTT!!!!!!!! NEXT!!
Well, based on this the Straw Hat Crew is missing the number 2,9 and this fits with Nika.
Also, a bit more info:
- Gomu = 56, Nika = 29
- 2 looks like an upside-down 5
- 9 looks like an upside-down 6
- Roger to Rayleigh “let’s turn the world upside down”
The abilities of the Nika Nika No Mi is still rubber but there are additional attributes like liquid rubber or resin (a popular theory) once awakened.
The reason the World Government wanted to get rid of the name wasn’t because of its powers but it was due to the previous user who was Joy Boy aka Sun God Nika and they wanted to get of that name and any association from the history books.
We know that the World Government would do this, an example was when Who’s Who was in prison and a guard mentioned the Sun God Nika to him and how he freed slaves in the past. That guard disappeared the following day without a trace.
This fits the theory that the Will of D. is a smile/grin on its side and it represents liberation, and it also aligns perfectly with Luffy’s ideology as the Pirate King: the freest man on the sea.
An argument for the name Nika Nika No Mi would be how a smile/grin relates to rubber/elastic abilities. First of all, a devil fruit’s name doesn’t always relate to the power e.g. Nikyu Nikyu No Mi (paw fruit).
However, in this case, it can be explained that to smile, you need to stretch your face. The more you can stretch it, the wider you can smile/grin.
*Theory by 50thsun