I don't think the end implied Gabriel was stronger than Satan; only that he was stronger than him when Satan when on Earth. His bio says "However, he is not able to command his full strength while chained to the earthly plane, and thus remains weakened against Heaven's chosen." When on Earth, I think Satan has to rely on his cunning a lot more than his brute strength, so the boss of Reverie could have more raw power than Satan does when you fight him in the game.
oh, and i'll laugh if the boss turns out to be Walter and Gabriel captures his soul into a relic, transforming him into Dracula.