Instead of dying like a buster, Satan gets back up again. Gabriel's all "Huh?" and Satan's all, "Bitch,
I'M FUCKING SATAN, you can't just stab me to death you moron, check these pecs out".
Epic multi-stage fight ensues, Alucard fights by your side in some spots, and the developers add in i-frames to the dodge move in the same patch so you can stay all up in Satan's grill. If you die, you see a cutscene of Satan pulling Gabriel's face off like he did to the Acolyte (God that was awesome).
Only after some Asura's-Wrath-level QTEs does Satan finally succumb, and Gabriel, exhausted and knowing his work is finally done, gives the nod to Alucard to do some Vampire Killing. Touching speech, tasteful redemption stuff, yada yada Gabriel dies.
Maybe Alucard dies too and the family is reunited in Heaven? Or Alucard wanders away, finds that Victor Belmont didn't actually die in that dumb scene and then they bro out in a post-credits thing? I don't know, I mostly just want to fight roided-out Satan