I feel that Gabriel as Dracul makes perfect sense, in a "christian" sort of way. If you pay attention to what many of the characters said to Gabriel during the game, particularly Carmilla and Pan, you can see how all this was alluded to.
With Carmilla, I think it's three things which cause her to say what she does concerning Gabe's fate. First, she knows that Gabriel is being used. Does she know by whom? Maybe, maybe not, but she probably at least suspects Zobek. Second, she's sees that Gabe actually has the potential to do what he says. That is, destroy them all (even Zobek). Third, she is acutely aware of TFO's existence, and thus knows that if Gabriel does succeed in his quest, the only way for Gabe to keep the world safe is to stop TFO by becoming a vampire himself, thus why she says "you'll be one of us, eventually". As I see it, Carmilla was acting as a sort of gatekeeper for TFO. Hence why Olrox kept saying "the dark lady watches over you". He was referring to the fact that Carmilla was using he power to keep TFO at bay (so even though she was evil and wanted to subjugate the world, she still uses he powers for good in a way)
From Pan's end, it seems as if he foresaw it in a vision or something, asking Gabriel if he could "accept his fate". Gabriel being "fated" to be Dracul though, means that God lead him down that path, which may seem confusing to some. In a world where freewill reins, both good and evil are equally possible, otherwise it wouldn't be freewill. And though much evil happens through the abuse of our freedom, God seems to somehow hold it in check, even if sometimes only to the point of just barely holding back complete destruction. For example, the Bernhard family, through an abuse of their freewill, were able to summon a force so destructive it would've lead to the annihilation of everyone on the planet. To counter this, God uses the Brotherhood of Light to seal The Forgotten One away. Unfortunately, the founders, in their ignorant pride (which is what initially lead to the release of TFO by the Bernhard's...ignorant pride that is) became at least in part evil themselves, and thus yet another blight on humanity.
So, when all hope seems lost, in steps Gabriel to save the day. But poor Gabriel doesn't know he's being manipulated at every turn. Zobek thinks he's pulling the strings. Using Gabriel to do what he couldn't, which is to kill his rival Lords of Shadow leaving Zobek to rule the world w/o rival, while simultaneously acquiring the pieces of the GodMask, something unattainable to any of the LoS. But then we find out that Zobek was just being used by Satan, possibly for the very same purposes. The GodMask may have seemed to be Satans only motive, but if Zobek doesn't want the other LoS around as competition for Grand Evil Head Hancho, I can guarantee you the Devil wants it even less. But Satan isn't the only one with a plan behind the plan.
Just when all seems lost, God brings Gabriel back to life (Gabe did ask after, "Why has my life been given back to me?", which means that he was indeed dead after Zobek zapped him), and uses him to open up a can-o-whoop-ass all over Satan's behind. After finding out that Maria is still going bye-bye (though at least she'll be entering into the Lord's presence, so it's not all that bad), Gabe feels incredible remorse about what he's done, even if unbeknownst to him while doing it. I mean, if you woke up tomorrow to find your wife dead, would it ease your suffering to know that you killed her youself, in an unconscious state, or would that make it worse? I'm guessing worse. So I'm guessing Gabe feels more as empty and confused as any man ever has. He has no idea what to do with himself.
Then Claudia calls to him, and gives him the only reason to continue to fight that could possibly spur him on. The absolute destruction of all of mankind. The Bernhand family brought TFO to earth. God uses the Brotherhood to lock him up. Years later a new threat appears in the Lords of Shadow, and for a time God uses to Brotherhood itself to fight them. But that battle, fought for centuries, eventually goes sour, and has lead to what would be the eventual complete destruction/subjugation of all mankind. So, God uses Gabriel to keep that from happening. Gabe destroys two LoS, Satan destroys the third (or at least certainly kicks him down a notch), then Gabe beat Satan. Then, finally the final threat in TFO himself. Sure, Satan was pulling the strings on Zobek, who in turn was pulling the string of Gabriel (a bit), but ultimately it was God who was pulling the strings of them all. Not with puppet like control mind you, but simply by allowing them all to go the way their freewill was leading them. Even using evil itself to do His will.
I think Tolkien puts it best in the in the Silmarillion when he wrote Illuvatar (God) saying to Melkor (the devil):
"And thou, Melkor, shalt see that no theme may be played that hath not its uttermost source in me, nor can any alter the music in my despite. For he that attempteth this shall prove but mine instrument in the devising of things more wonderful, which he himself hath not imagined."
Even in Gabriel becoming Dracul, God's will is done (perhaps not His perfect will, but His will in accordance to allowing freewill). As powerful as Dracul is, being now the absolute lord of evil on earth, I suspect he still has a part of his humanity in him, buried deep, which is one of the reasons why God makes Gabriel Dracul in the first place. Whatever Gabriel's origins, God's plan for him all along was this very thing. Just look at Gabriel at the end of the epilogue. He's tired, not just physically, but mentally and spiritually as well. Just like Claudia before him, he feels the goodness of his humanity calling out to him, and wants release from being the wretched thing that he his. I suppose that's what forgiveness and salvation are all about. Coming to the understanding of how truly wretched we are, and desiring to be free from our sinful selves.
Gabriel as Dracul may be terrible, but even with all the power of the LoS combined, plus the added power of TFO, he will be a lesser evil compared to what would have been otherwise. He is a threat to the world, but he's a threat that because of his ingrained personality, will not destroy the world. He is the reluctant lord of darkness, whom God will use once again to defeat Satan a second time.
And so, Pan see's Gabriel's fate. How much of that fate we don't know, but at the very least he foresaw that Gabriel was to become something terrible, and through that to save humanity from something far worse.