All the AI for SAO would actually come from real people who volunteered to be inside the game as AI. Gilgamesh would be a computer hacker that would want to get into the development of SAO and Yui would be a poor orphan that lived in the streets, and got added as an AI because Kayaba pitied her. All AI original people would die after being scanned inside of SAO. Gilgamesh would take care of combat AI, and after seeing Kirito combat skills and how Yui managed to go out of her asylum, he does the same and constantly challenges Kirito for battles. Capable to morph into a "boss form". Later on Gilgamesh would pop up in ALO when Kayaba was about to give the world seed to Kirito, and get some of it's code, allowing him to travel at will to any game that uses the world seed in some way.