But that's where you're wrong. No matter how bitterly he was betrayed, no matter how maliciously he was manipulated, and no matter how regretful of his plight he might now be, Gabriel knows that he can no longer escape what he has become. Dracula is a creature of evil, of hatred, of pure despair, rebelling against the very deity in whose name he once fought. The blind love which once guided him is now naught but a shadow, the insubstantial reminiscence of a passion he can no longer harbor -- all of it has now turned into the inexhaustible rancor and scorn that so defines Dracula's character. Gabriel knows that there is no going back; he knows all too well that there can be no redemption for him. In the end, that is the terrible truth that he so loathes to admit, and the true essence of LoS2's tragedy: he will blame everything for his abhorrent fate, except for what truly brought it about: Gabriel himself.
Except Gabriel did NOT bring it on himself. He was following orders. he was doing his duty to the Brotherhood. It was prophesied that the Lords of Shadow were the key to ending the dark spell over the land, and that by getting the pieces of the God mask from them, he might be able to resurrect his wife. Marie, pan, and the Brotherhood all lied to him and misled him however, so that he would be unaware of his ultimate fate.
Even leaving out his rage and pain at not being able to revive Marie, and Satan being the mastermind behind the whole plot, with Zobek stringing him along, In killing the Lords, he unleashed the Forgotten one and weakened his seal. He killed the Lords, so he was responsible for bringing about a creature that could decimate the world. And the only way to enter it's realm to stop it was to become a Vampire, and lose his Humanity. The DLC very clearly shows his progression, as he turns, he slowly becomes colder and more arrogant until he is declaring himself "The Dragon".
It's hardly his fault at all. After that however, there is the matter of his blind rage towards the brotherhood for doing that to him, and the unfortunate concequences of their antagonism towards him.
They hid his son from him and sent him to his death,
They caused Simon's mother to be killed during the attack on the Brotherhood, carried out in vengeance for the death of his son
They caused Simon to also wish vengeance upon his father's killer, not knowing the circumstances.
It's all a viscous cycle.
Besides, the whole point of LoS' epilogue, and the prominent modern day theme of LoS2 is that Zobek bargains for Gabriel's help in stopping Satan again in exchange for releasing him from his immortality. Which he CAn do, he IS death after all- Although I'm willing to bet that it's not Zobek which will release him in the end.