1- Jinbe did it, against Who's Who for example.
2- Now that you made me remember: Lucci did it with Rokuougan. Ranged Shigan and ranged Rankyaku are just that as well. Zoro's slashes are just that.
3- Humans can use Fishman Karate.
My point is literally there. Koby wouldn't be the first one to do that, and using the excuse of "well but he didn't train those techniques!" is a skill issue, he would remain not the first.
Still, if he did anything similar to Garp's moveset, it will be haki based, quite possibly CoC based (if it were pure strength, it wouldn't have anything to do with Garp's technique outside your mind), but it will still be a shadow of Garp's move. That much is obvious for anyone that isn't blinded by hype for their fav.