20The True Purpose Of Shield Of The King
The connection between Gilgamesh and Ardyn's backstory was only hinted it at in the full version of the game, though fans had speculated that they were linked.
The Final Fantasy XV version of Gilgamesh had a lot of cut dialogue that was discovered by the fans, some of which answered several questions as to his purpose.
The dummied dialogue revealed that Gilgamesh created the role of the Shield of the King specifically to protect the current
monarch from Ardyn.
Gilgamesh was around during the life of the Founder King when Ardyn first succumbed to evil. This led to Gilgamesh sealing his own soul away so that he could pass on his powers to a future Shield of the King.
Gilgamesh's dialogue was likely removed in order to lessen spoilers for people who hadn't finished the game.