In One Piece, Luffy can use gears 2-4 (I know from a little bit of spoilers, I'm on the Water 7 arc and he has already used gear 2), but so far I haven't seen another devil fruit user use gear like Luffy's. Is there any devil fruit user that uses gears?
You're asking the wrong question. Gears is just the name given by Luffy to the extra usage of his unique paramecia body, hence technically only he can use Gears.
Luffy uses his Elastic Heart and Blood Vessels to increase his bloodflow, this allows him to increase the limit of his body. He can move very fast and increased strength. It however decreases his endurance as he experiences more fatigue
Luffy pumps air into his bones, which also being of rubber expands and allow him to increase the size and momentum of his moves
Here Luffy pumps air into his muscles. This allows him to use Haki, while remaining elastic enough to dodge/bounce back attacks
Other users have their own special moves. For Example:
1. Magellan - Covering his body with a stronger poison (Venom Demon: Hell's Judgment)
2. Robin - Using Thousands of Hands to create larger body parts including Wings. (Gigantous Mano or something)
3. Mr.3's Wax Suit (Champion Form)
If they call their special moves something else instead of Gear N, it doesn't change the fact that they are using there fruits' ability in a new way :)
NOTE: All Credit to Luffy for thinking outside the box, though I doubt he considers the impacts of those actions before doing something, usually he does it because its COOL! :D
source : http://onepiece.wikia.com/wiki/Paramecia; Anime/manga
Edit: added more examples.
Edit 2: With the introduction of awakening, it makes sense to add that each fruits awakening will also be unique. Thus Luffy's version of awakening and how he uses it or even accommodates it into his Gears will be upto his ingenuity. (Personally, I want him to use a Staff (Like young Goku), which can extend due to his Rubber powers!)
Luffy is the creator of Gears, and so far in both the manga and the anime, he is the only known user to use it. In the One Piece wiki, the only person who can use Gears is Luffy, and the only reason he can even use gears is because of his Devil Fruit, the Gomu Gomu Fruit. Wiki quotes:
By taking advantage of the unique physical abilities granted by his Devil Fruit, Luffy has invented techniques that augment his speed and strength in different ways, though each one also has corresponding physical drawbacks. He refers to these techniques as Gears.
The way Gears work (you will see quite soon because they explain it in the CP9 event during the fights) is that Luffy pumps his blood a lot faster than normal, and his veins don't explode because he can stretch any part of his body, whereas if a non-stretchy person tried this, their veins would explode causing instant death. Therefore, no one besides Luffy can theoretically learn Gears.
Gears is just a name to his techniques. They are called gears basically since he is forcing his body or devil to its current full potential. If we go with that thinking then we can say that logia devil fruit user also use "gears" when transforming to liquid (well not liquid but when they turn to light or sand or fire etc). The one thing that all devil fruit users do have is awakening. But since you are in Water 7 arc I'll just stop here.