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.
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 Worldof 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.