Seems like certain characters play in certain parts of the game.
Also, Im sure that there are records of the good Gabriel's done, he might be recorded as the guy who saved the world but then fell from grace and became a vampire overlord. Or just a good brotherhood knight who fell to darkness.
BUT THEN AGAIN- NOBODY KNEW WHO THE FUCK ZOBEK WAS, NOT EVEN BY NAME, AND EVEN HIS RECORDS DIDN'T KNOW HE WAS A FOUNDER, SO I DUNNO, THEY HAVE A SPOTTY TRACK RECORD AS IS...
I'm guessing Gabe will be in the deep of his anger an hatred and offer Trevor to Join him as father and son, ala Darth Vader in Star Wars ll, and Trevor will say he cannot follow him into Darkness and thus they fight. At whatever point which Gabe says "You fight well, worthy of the name Belmont..." and similar stuff.
How will GABE react? Now there's a question... probably make him angrier at the Brotherhood for keeping it from him. I doubt he would blame Marie.
Im going to guess Trevor loses though, And therefore that's where Simon comes in, and Simon, along with Trevorcard, set the record straight.
And yeah the lost soul will do something I guess.