Because there really aren't any "Dragon Gates" in the Pokemon world.
Caesar was lounging around in his home in New Bark Town one dreary Spring afternoon. The weather wasn't bad, but it wasn't quite to his liking. He had a fondness for the warmer aspects of the season, and not much else beyond resentment for the cloudy days which came with the reminder that there may just be rain on the horizon. His boredom caused him to put aside his disdain for swimming, deciding he would swim out to Tohjo Falls for some exploration to pass the day away. Getting up, he packed his waterproof backpack lightly; a lunch for himself and his Cubone, a couple of Pokeballs and a change of clothes for when he got there.
Smiling to himself and thinking of the coming adventure that awaited him at the falls, he waded into the water and began making his way across the brisk waves. Being a strong swimmer, it didn't take him long to reach his destination. Five or ten minutes, maybe. Climbing up onto the rocky shoreline, he checked to make sure no one was around before changing behind a tree.
Being content to have gotten out of the relatively cold water, he made his way inside the cavern before him. There were legends - folktales, really - that any Magikarp captured here would become a fearsome Gyarados. Of course, that applied to any Magikarp; it was common sense, really, but he remembered the story from his childhood. "Come on out, Flint!" He said confidently, tossing the Pokeball into the air.
The capsule broke open, unleashing a brilliant crimson light that took the form of his beloved Cubone. Hearing the sound of a cascading waterfall, he made his way in the direction of the noise; his Pokemon alongside him. The walk there turned up very little in the way of excitement - a Geodude here and a Zubat there - though nothing seemed to mind his presence. Reaching the lake, he sat down on the shoreline, taking his backpack off of his shoulders and unzipping it. "How about some lunch?" He asked his closest friend, who responded with a monotone "Bone". Flint wasn't one for saying much, which was more than fine with him; he rather liked the silence, himself.
As he went to hand Flint his lunch, he heard a splash come from the surface of the dark waters. Looking up, he saw a Magikarp jumping from beneath the shadowy depths, glinting in the air. Was it... Gold? Surely he didn't have the luck to encounter a Shiny Pokemon? "Flint, forget lunch for a second." He said, returning the food to his backpack and getting up before diving into the lake. He swam after the Magikarp, which easily evaded him in it's own turf.
Giving up after around an hour of scouring the lake, finding it and then giving chase to only lose it again, he gave up. Swimming back to the shore, he climbed out of the water; only to have the Magikarp use Tackle on him from behind, knocking the two of them onto the rocky floor of the cavern. Standing up and brushing himself off, he pulled a Pokeball from his bag; knowing a fish out of water was the easiest fish to capture. "Go, Pokeball!" He yelled with excitement as he threw the spherical capture device at the flopping fish. It burst open, the red light bathing the Magikarp before pullling it inside. One. Two. Three. A hollow "ding". Picking up the Pokeball, he gathered his things. "Let's head outside. I want to check out our new friend in better lighting." He said to Flint, the two of them heading for the entrance.
Once outside, he tossed the Pokeball that held his new Magikarp toward a small, seemingly enclosed lake. "Let's take a look at you, Magikarp!" The ball burst open, and once the light sparked away from the form of his newly caught Pokemon, it revealed... An ordinary Magikarp. "Well, let's eat." He said to Flint and their new companion with a chuckle.