In ancient times he roamed, traveling from place to place with little a goal. His master abandoned him, and he was set free, but to what end? He wandered and wandered, with his faithful companion to keep him from falling into despair. After many many years of wandering, they eventually stumbled onto a room, where he found a man. The man was gazing into another realm where there were many involved in a great conflict. The traveler asked the man what the realm was. The man replied that he was a scientist, one experimenting with the very roles of the universe. The traveler looked inside. War clashed, men were slain, and carnage was abound, but in there, the traveler had a sense of longing, as though that was where he could be needed.....and happy. The scientist could tell what the traveler wanted, and though he advised against it, allowed the traveler to join in the experiment. He warned the traveler that he would have to be separated in order to fit in the boundaries of the world, but the traveler didn't care, for he had an overwhelming desire to fight with and alongside the denizens of that world. And so he was separated. His kind nature was alienated, becoming malleable, and it took form of one whose was blessed by the wind. His skills and drive were torn from him, and took the form of a warrior whose goals would never be stomped out. And finally, the warriors strength and adaptability grew at an astonishing rate, taking a form befitting of a warrior's soul. And into the world he fell, separated into 3 pieces so that the whole may find joy.
And that my friends, is why Gilgamesh isn't in Dissidia