With the recent revelation of Law and Kid's fruits being awakened, it's very possible that Luffy's is as well - and one piece fans just never knew.
Ever since One Piece's Dressrosa arc, fans have wondered when (and what) Luffy's Devil Fruit would awaken. First introduced through antagonist Donquixote Doflamingo, as strong as Devil Fruits can be, they all have the potential to evolve and become even stronger. This process is called "awakening." First shown off by Doflamingo and later elaborated on by Katakuri, their awakenings allowed them to have complete control over their environments.
In Chapter 1030 of the One Piece manga, readers were shocked to find out Eustass Kid and Trafalgar Law had also awakened their fruits, albeit not with the mastery level that Doflamingo and Katakuri had. While their abilities did grow, Kid and Law both stated that using their awakened abilities would drain their stamina greatly, putting them in a vulnerable state. It might be a coincidence, but the drawbacks to using their awakened abilities are quite similar to what happened when One Piece Luffy first created his different gears.
First unveiled after the Straw Hats made it to Enies Lobby, the Gears were notable as the first real power-ups Luffy had ever had in One Piece. With Gear Second, he increases his blood flow by using his rubber body and cells to force more blood through his vessels. Doing this greatly enhances Luffy's physical abilities to the point that CP-9 agents would have trouble keeping up with his speed and strength. Though it's a great enhancement, relying on it would drain his body of its nutrients, weakening him significantly. Rob Lucci pointed out how the process was similar to doping, and could decrease his lifespan if used too much and incorrectly.