The Gomu Gomu no Mi is a Paramecia-type Devil Fruit that gives the user’s body the properties of rubber, making the user a Rubber Human.
One of fans asked Oda why he give Luffy Devil Fruit’s weakest power how rubber, and Oda answered that give him funniest power, that no matter how serious the story in the manga was, the reader would be able to relax and observe with interest what is happening:
D: “In the ONE PIECE of these days, all kinds of strong ability users whom even Luffy can’t compete with have been appearing, one after the other. In general manga and anime, I feel that the protagonist is established to be the strongest of all, but why did you decide to turn Luffy into a pretty weak-looking rubber Devil Fruit user? P.N. Do your best Japan”
Oda: “Ahaha, that is true. It must seem like a mystery when you consider all the other strong and cool abilities. The answer is simple. I have picked the most ridiculous ability. If the protagonist were the typical strong guy, I doubt I could continue on with this for long. No matter how serious the story gets, Luffy is there to stretch and inflate. He always give me a chance to fool around. That’s the kind of manga I wanted to write.”