Quick answer
When did Luffy eat his devil fruit?
Verified through chapter 1Updated
What the manga shows
3 factsLuffy ate the Gum-Gum Fruit taken from an enemy ship.
Luffy shows his rubber powers after eating the fruit.
Luffy later states he is glad he ate the Gum-Gum Devil Fruit.
Change log
1 chapter- Chapter 1Added 3 facts; updated the answer