My guess:
Gargos was part of a pantheon of gods, formed by powerful celestial beings who acted as gods to the humans. Gargos betrayed them somehow and got corrupted. Maybe he was similar to an angel, like one of his sets hints. He is somehow the “Devil” of this universe.
Yeo, the dragon, was surely one of these gods. Maybe the Tiger Spirit was one once, but Gargos took his identity for his own benefit.
Surely this lead to a war, and the gods choose champions. One of Kim’s ancestors was Yeo’s champion, and maybe all of them forged a living weapon against the shadow lord: Tusk, the Watchmen of the Gods, who will be the eternal gatekeeper, not allowing Gargos to return from the Astral Plane, where he was banished after the war. Maybe the Tiger Spirit champion was there too, and Gargos had acolytes too, maybe the Tsar. Gargos is surely the father of all types of abominations, like werewolfs, vampires, wendigos… When Gargos lost(or even before), the Night Guard was born to destroy his creations
There are many relics that could be Gargos or other gods crearions, like the mask used to curse Spinal, Temperance and Vengueance, or the tome Mira took from Sabrewulf. Even the Stone used to infuse life in Golems like Aganos, which him is wearing in his head.
Other champions could be beings like Hisako, choosen to defend the Astral Plane barriers, not allowing the Shadow Lord or his minions to come back.
IMO Gargos manipulated all this. He used his influence from the Astral Plane to seduce Kan-Ra into betraying the Babylonian King, being then Cursed. But this was part of a bigger plan: Tusk was there too, and he was framed into Kan-Ra’s conspiration, so the King also punished Tusk. His curse wiped his memories, making them unstable, so when the time come, Tusk will not be there to deny his return. The objective of Gargos was multiple: He knew Kan wouln’t die, and he will be pursued by Aganos. Eventually, Kan-Ra will learn about the Astral plane, and manage to overcome Aganos, opening a rift which Gargos will use to leave the Astral Plane.
And IMO he has something to do in the creation of Ultratech, but I’ll keep that to me.