Gabriel Van Helsing is Professor Abraham Van Helsing's long-lost sonIn the book Dracula, Professor Van Helsing mentions that he had a son who died, but doesn't go into detail about this. In this movie, Gabriel Van Helsing has no memory of his past or his family. Granted, the movie implies that Gabriel Van Helsing is the human form of the archangel Gabriel, but he still could have taken that form by being born as the professor's son. This theory would explain why this movie's Van Helsing is a generation younger than the book's, why he doesn't exactly have the same profession, and why he has a different first name but the same last name.
- Alternatively, with the implication that Gabriel is an Angel Unaware, Abraham is essentially his adoptive father and is/was a very high-ranking member of the Holy Order, and probably taught Gabriel (and Carl, for that matter) everything he knows but is too old and frail to be on active duty at the time the film takes place. He's probably off helping the Harkers (who may well also be members of the Order) investigate/deal with some other threat.
Van Helsing takes place in the same universe as the 2011 adaptation of The Three MusketeersBoth films seem to take place in an alternate history, that's slightly more badass and politically correct than our own. They both involve high-tech weapons and spy gadgets (and airships, in the case of the latter) made from technology that matches the era they take place in, except for being insanely badass. Both have lenient views on women fighting alongside men; neither Anna Valerious nor Milady had to prove herself capable of fighting or intellect their societies or their male colleagues, it was just accepted. And both are populated by badass heroes and over dramatic villains, of course. The Three Musketeers (2011) merely takes place early than Van Helsing, by a couple of centuries.
Van Helsing takes place in the same universe as The Mummy TrilogyMummies were not represented in Van Helsing, but the first two Mummy movies have the same director, and they're all from Universal Studios. The Medjai aren't represented in Van Helsing's organization because Imhotep was the first mummy to wake up ever. (This does lead to All Myths Are True.)
The first werewolf we see taking on the Valerious siblings is...Lawrence Talbot. Just a little fan theory, plus he was the only member of the iconic trio of Universal Horror monsters missing from the movie.
The Movie is set in the Old World of DarknessAnyone familiar with the Vampire and Werewolf settings would immediately spot a knock-down drag out fight between a Shadow Lord Ahroun and a Tzimisce vampire using the winged variation of Zulo form. Right down to the original description of how razor/silver claws worked (You scrape your claws on something hard to sharpen them.) Everyone just thinks being a werewolf is an infection, not a species. Gabriel's just a late changer, is all. In addition, he steps through a mirror to travel.
Gabriel asked to forget.The eons of slaughter even for a righteous cause began to wear on him. More and more he longed for peace especially after the incident with Dracula. Eventually he pled with God to grant him respite from the eternal vigil. God agreed but with the condition that Gabriel would not be able to remember what divinity was like nor his life in it. However once he was more human then not he couldn't help hunting the memories lost, all according to plan actually and one day when his respite is at an end he will return to his status as an archangel, but for now the small scale work of monster hunting is a worthy hobby.