I just finished watching the entire walkthrough on youtube and feel that I need to share my crazy opinions...
Quick comment about the game : the level design, rivals and events only became interesting to me after the first third of the game (after Cornell)...
As for the storyline, here is a script excerpt after the fight against Satan (comments in bold) :
Gabriel: I am alive? No, I do not want this. Why has my life been given back to me?
Marie: It is your fate. You have been given back what was wrongfully taken from you. To repent your sins. To make amends.
(My guess is that this is understood as a divine punishment for Gabriel. To stay alive in order to repent, and constantly feel the pain of not being with Marie. That would be enough to hold a grudge against God)Gabriel: But without you..?
Marie: You freed them all, you saved us all, my Gabriel.
Gabriel: I couldn't save you.
Marie: I knew I could not tell you or despair would have eroded your resolve and everything would have been lost.
(So actually she didn't fully trust him or his good heart... that's a cruel betrayal from the one you love)Marie: I had faith in you, hoped that would be strong enough to free the world and you did not let me down.
(This part of the script was added to keep the player's mind away from what I've pinpointed in the previous sentence)Gabriel: I am not worthy of your faith, your love... I am nothing.
(Notice how Gabriel can't seem to focus on the positive outcomes of all of this. He keeps thinking about his pain and negative feelings)Marie: You are a good man Gabriel. You are as God intended, fallible yet capable of great things. I loved you then as I love you now. I see before me a man who has regained God's favor and who has my forgiveness and the forgiveness of all the lost souls in this world. You have saved us all and... you have saved yourself.
(Still trying to convince him that everything's fine...)[Gabriel reaches for the mask]
Marie: The mask is a powerful device. It allows us to see through God's eyes.
Gabriel: Can it really bring the dead back?
(See? He doesn't give a damn about having saved the world. He only wants to be with Marie again)[They kiss, she takes the mask away]
Gabriel: A fool's hope...
Marie: It is too late for me my love. I cannot go back.
Gabriel: No... no, don't go.
(Notice how she approaches her hand to take his, but he holds out his "gauntleted" hand and she backs away)Marie: It is my time, the light is calling. I'm not afraid.
Gabriel: No, please stay. Stay with me!
Marie: It's beautiful Gabriel... so beautiful... It's calling to me... calling
(These words make him feel that she is taken away from him. Grudge against God rises over 9000 )Gabriel: I love you Marie, I love you... forgive me.
[She vanishes, Claudia appears and vanishes too, Gabriel falls on his knees, looks above and cries. Then the camera freezes on Zobek's mask. Credits roll.]
And after that is the ending in the 21st century where Gabriel is Dracula and seems to live as a hermit and doesn't harm humans.
So why did he become a vampire and became known as the Prince of Darkness ?
Maybe Gabriel used Zobek's mask to become a vampire, or was bitten by one (could be a normal enemy or Laura). He had to make things such as ruthless ruling, war or dark magic stuff in order to become known as the Prince of Darkness. He learned romanian at some point. "Eu sunt Dracul" could translate as "I am Dracula" or "I am the Dragon".
As for Zobek, it seemed pretty obvious that he died in the game, burned by Satan. So why the heck is he still alive? He wishes to fight Satan. For Gabriel, joining the fight would also mean fighting for God...
Dracula: I cannot die, yet I cannot live.
Zobek: Help me and I can free you of your immortality
(So basically Gabriel has to be used by and rely on others again in order to find peace and happiness)[Camera zooms onto Dracula, he seems to be giving a thought about the situation, but shouts and vanishes with bat screeching sounds]
Zobek: Soon old friend, soon it will be over.
Judging from their looks, Dracula has the casting of an antihero and Zobek that of a boring helper/rival. Based on that it would not be surprising that a sequel would feature Dracula as the playable character in a Legacy of Kain-esque game.
If the game is truly a reboot of the Castlevania story (like the new Batman movies), Gabriel is not Leon Belmont's ancestor and most likely never fell in love again and never had children. There is no Vampire Killer whip made by Rinaldo's alchemy and Sara Trantoul's soul. Mathias Cronqvist most likely never existed. So there's no point in assuming the previous Castlevania characters and events are part of the new storyline.