The religion in the game has the words "All humans shall be treated equally" written in them. These words are assumed to be from Gaius (the god) himself. The religion that formed around him used those words to justify slavery of elves.
In this story, I've written the religion to be corrupted by humans. This has nothing to do with Gaius or any God in real life, or if they exist, but is to show how imperfect humans are.