Knowledge of Gear 4th through the world In the chapter 882 of One Piece ("Beyond the Expectations of a Yonkou")


   We see Luffy at the moment he'll use his Gear 4th, trapped in the Flowing Mochi, and his opponents reacts in this way: 


This assumes that he has knowledge of both Luffy's Gear 4th technique and the mecanism to activate it. However, Luffy never mentionned it in his fight against him, so it is the first time he sees both Luffy and his technique.

How so ? Is it something that is known over Devil Fruit users, and everyone who ate them has "gears" like Luffy ?
You forgot that Katakuri can see few seconds into the future with his observation haki, so he already "saw" Luffy transform into his new form.

It has nothing to do with other fruit users, so far Luffy is the only one who can use "gears" as a transformation mechanisms and it will most likely remain like that through the series.