Gyarados is really good during Johto just like it is in Kanto. You can catch Magikarp with an Old Rod at the first gym city (Violet City) and evolve it to Gyarados at level 20 and onwards. It doesn't matter too much what nature Pokemon have when it comes to story, but Adamant is the best nature on Magikarp/Gyarados. A good Gyarados moveset for story is Dragon Dance, Aqua Tail, Crunch, and Ice Fang.
I would also recommend catching a Rattata and leveling it up during the Johto story (but not evolving it), since you will need a level 80 Rattata with your Original Trainer ID in order to enter Hoenn after the Johto Elite 4. Also, Sucker Punch Rattata helps a lot at the fourth Johto gym.
And i ask , attractive pokemons in johto to start level up since i arrive johto? and goods natures?
thank you for helping me