Gomu Gomu no Mi

The  Gomu Gomu no Mi  is a  Paramecia -type   Devil Fruit  that enables the user's body to stretch like rubber, making the user a Rubber Human  (ゴム人間 Gomu Ningen). "Gomu" means "Rubber" in Japanese. In the Viz Manga, 4Kids and FUNimation dubs, it is called the  Gum-Gum Fruit. It was originally a treasure that  Shanks  and his  crew  took from an unspecified enemy, but was accidentally eaten by the series protagonist  Monkey D. Luffy. In the second version of Romance Dawn, it originally belonged to Luffy's  grandpa  who took it from an unspecified enemy.

Appearance
Luffy struggled to control his Devil Fruit powers as a child. In his youth, Luffy had a problem controlling his own abilities. Though he had an idea of how they would work, Luffy had trouble with aiming and the timing of his attacks, and he was continuously sent spinning by the recoil. However, he managed to counter these weaknesses with his continuous training in the jungle by the time he left to be a pirate.

In the anime, Luffy has used his stretching abilities to inhale great amounts of air to thus exhale a big gust of wind; this application of his powers was used to blow away Marigold's poisonous vapor from her venom spit and Magellan's poison breath.

Given Luffy's elastic properties he is shown to be able to withstand great pressure, as shown near Fishman Islandwhen fighting Hody. Though he was still weakened by the sea water around him, he was seemingly unaffected by the intense underwater pressure.

Luffy, of course, also suffers the same weaknesses to water, like Fishman Karate and Kairoseki, as other Devil Fruit users. However, he sometimes appears to forget about his weakness to water, as he always wants to try some type of water sport in the sea and will often dive into the ocean when he sees someone drowning (only to end up drowning himself). And while contact with water and Kairoseki does drain Luffy of his energy and nullify his rubber-like ability to absorb blunt attacks (as with Smoker's jutte), his actual body still maintains its rubber-like make, allowing a third party to stretch his body for him.

The Gomu Gomu no Mi user is also susceptible to Busoshoku Haki. Similar to water's effects, the usage of Haki only nullifies Luffy's ability to absorb blunt attacks. It does not however, revert Luffy's body to unstretchable flesh. Through the use of Busoshoku Haki, Luffy's grandfather was able to hurt him using only his fist (though Garp attributed this to "love").

Usage
Luffy uses his Gomu Gomu no Mi abilities to power a fan-propelled boat.The powers of the fruit have been used by Luffy in a variety of ways. These range from simply moving from one place to another in a rather reckless manner, to attacking an enemy with a barrage of fast and strong punches. He has also developed several peculiar methods of combat as time passed, adapting new forms by manipulating his elastic body, most prominent his "Gear Second" and "Gear Third" forms, allowing him to momentarily increase his strength in combat. After the timeskip, he has learned to use Haki in conjunction with the fruit's powers, as well as having more control over his forms.

Of course Luffy, being who he is, does not only use his ability for battle. When with his crew, or even when partying with others, Luffy tends to use his elastic properties for the humor and joy of others, such as making funny faces, or using his stretching body parts to play around. Often throughout the series he would stretch out and steal other peoples food, much to their annoyance. He also uses it for utility functions, like transport, for example using his "Gomu Gomu no Rocket" to travel great distances quickly. Luffy also has no problem letting other people play with his stretching body, which people tend to take advantage of.

Early One Piece
Luffy's stretching powers were a result of Oda's desire for the fighting style to be silly, so that no matter how tense a situation got, the reader could relax without becoming stressed out. Oda originally just called the fruit the 'Gomu Fruit'. But his editor pointed out there really existed a Rubber fruit so Oda changed the name.

Oda rewrote several details about the Gomu Gomu no Mi in his early version of One Piece Romance Dawn V.1 and Romance Dawn V.2. In V.1, the Devil Fruit was the same except the Gomu Gomu no Mi was the fruit of the Gomu Gomu tree which grew once every 50 year and there were no named attacks in V.1. In V.2 the first named attack is given, however rather say "Gomu Gomu..." at the start of all his moves, Luffy simply said "Right hand stopper!". Little detail was given on the Devil Fruit itself in V.2.

Translation and Dub Issues
Viz, 4Kids and FUNimation all translate this fruit's name as the "Gum-Gum Fruit". This translation comes from the etymology of the Japanese word "Gomu". "Gomu" is a loan word from English, it comes from "Gum" which is used in English to refer to rubber to identify it as "Natural Rubber" instead of "Synthetic Rubber". This is not only true of Japanese, but most worldly language derive their word for "Rubber" from "Gum". For example, in Spanish and Catalan: Goma, French: Gomme, German: Gummi, Albanian: Gomë, Serbian: Guma, Hebrew: Gummi, Italian: Gomma, Finnish: Kumi, Norwegian: Gummi, and so forth.

Viz most likely selected "Gum-Gum" as a translation over "Rubber Rubber" due to "Gum" being phonically similar to "Gomu". The only English version of the series to ever translate it out as the "Rubber Rubber Fruit" is the Odex version. This is because it has no connections to the North American versions at all. Despite translating Gomu as "Gum", all English versions refer to Luffy as a "Rubberman".

Trivia

 * In the 3rd Movie, it is shown that Luffy will not sink in water in his Gomu Gomu no Fusen technique.It is also seen in the anime.
 * Luffy's first demonstration of his ability and his reason for eating the Devil Fruit differ between the manga and the anime. In the manga, Shanks grabbed Luffy's hand while Luffy was walking away causing his arm to stretch, with Luffy later claiming to have eaten the Fruit thinking it was dessert. In the anime, Shanks witnessed Luffy eating the Devil Fruit out of anger and tried to make him spit it out by grabbing the boy by the ankles and shaking him up and down, which caused Luffy's legs to stretch to the point where his face hit the floor.
 * Luffy has far more named Devil Fruit attacks than any other Devil Fruit user.
 * After the timeskip, Luffy can activate his Gear Second without doing his "blood-pumping" pose. He can now pump his blood while standing, and even select a particular part of his body to bestow the effect upon; this ability to centralize the effects of Gear Second to any part of his body is not unlike that of those exhibited by skilled Zoan users, who are able to transform only a designated body part into animal form.
 * After the timeskip, Luffy no longer reverts to Chibi form after using Gear Third, as shown after he used a Gomu Gomu no Elephant Gatling on Noah and did not decrease in size afterwards.
 * When Luffy uses Busoshoku Haki, his body darkens in color. In One Piece: Gigant Battle! 2 New World, the Busoushoku Haki give his body a dark blue color skin.
 * One of Oda's SBS questions asked if Luffy's penis could stretch, to which Oda replied that it could.