Gabriel Van Helsing is Professor Abraham Van Helsing's long-lost son
In 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 Musketeers
Both 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.