Why is everyone assuming that if the final battle is Gabriel vs. Dracula, we control Gabriel? Considering that since we are playing through the game as Dracula, it's more likely that we be controlling Dracula and fighting Gabriel. If that is the case, how do they separate? Many of you think that after the battle with Satan at the end of LoS1, Gabriel's good half went to heaven and his dark half remained behind and that is what became a vampire and killed the forgotten one. I'm not sure that's the case. From what I remember, after that battle with Satan, Gabriel was back to his normal self. Alive. The light that left him was the power the spirits gave him to come back to life and fight Satan.
In other words, I don't think that he never split into light and dark like the founders did. He just became a vampire so that he could survive the dimensional prison where the forgotten one was.