The problem with the Gabriel Dracula is that even when Gabriel is his most vile, it's always played off as a result of the inherent evil of mankind. Otherwise, Gabriel just sits in his castle and acts as if wants to just be left the fuck alone. He's just a brooding, "I feel sorry for myself" kind of Dracula. He feeds on humans, yes. But it's usually the ones in his own lands of Transylvania, ones that act to conspire with or against him, or the ones that actively march to take on his power (the holy church, members of the brotherhood, etc.). Sure, he tortured those individuals and acted horrible atrocities against them. You see evidence of that in the LOS2 castle. But it's almost as if he just didn't care and let the evil creatures he ruled over do as they pleased with the helpless humans that he or his minions came into contact with. Not because he was actively pursuing them out. I never got a real sense that the Gabriel Dracula was hell bent on dominating all of mankind or marching his armies of darkness across the rest of Europe and the world like some true evil dark lord. That is the big difference to me versus the classic Castlevania Vlad/Stoker Dracula antagonist. Obviously, the creators of the classic Castlevania Dracula took liberties and stretched the historical Vlad and Stoker Dracula as well. I don't think the historical Vlad Tepes had a desire to rule all of Europe or dominate mankind. He's even considered by some a hero in Romanian lore. Sure, he did very evil things, such as impaling his enemies and rival armies, torturing them; all the same, not all that more extreme than many other medieval rulers who used scare tactics to get their message across to rivals. However, mix in Bram's horror monster, vampire lore, with a touch of dark lord, and the Castlevania Dracula is essentially the Sauron of the Castlevania universe. Perhaps he has his reasons. But the fear factor against him is that you are the only thing standing in his way from him having complete domination over the Earth. You're not miraculously expecting him to remember that he was good at some point. Or that he has any compassion in him for anyone except perhaps his closest companions or relatives. And even that is really a stretch.
Edit: When I am referring to the more classic Castlevania Dracula, I am referring to the horror flicks / Bram Stoker / Vlad Tepes based one. I know that IGA / Metroidvania and the later Castlevanias before LOS changed that origin to Mathias Cronqvist. Though, uhm, I'd argue Mathias is a "wussified" Dracula too.