Gentlemen is an unusual book in many ways. Chabon said in the afterword that his working title was Jews With Swords for a long time. Chabon is not a genre writer, but rather a "literary" writer and a reasonably successful one at that (a Pullitzer Prize at a young age tends to do that to a writer). Despite that, he's also notoriously a defender of genre and plot-driven fiction, as opposed to the plotless character studies that make up much of the literary fiction market.
That doesn't necessarily mean that he's a genre writer himself, of course. In fact, in his afterword, it was difficult to tell if this book was pretentious slumming, or simple self-deprecation of sorts. I'll give him the benefit of the doubt. His style, however, was very literary rather than genre. I'd say an integral feature of most genre fiction is that the prose disappears. Clever turns of phrase and elaborate metaphors take the reader out of the story and away from the plots and characters. Granted, they can be appreciated in their own right, sometimes, but their applicability in what is supposed to be a historical fiction swashbuckling romance with vague hints of Conan and D'Artagnan (according to the author) and Fafhrd and the Gray Mouser (obvious from the characterizations of Zelikman and Amram) is somewhat more dubious.
Chabon's interest in the two characters as atypical Jews, as well as the Khazar state as one of the only states of the Middle Ages to support Judaism as a state religion is kinda interesting, but borders on an annoying affectation after a time. He made too much of a point of talking about how Jewish everyone was, which got distracting after a while, as well as begging the question of how historically accurate this whole thing was meant to be as opposed to a specifically Jewish swashbuckling romance (actually, that's rather easily answered given that he features the khagan Zachariah, who actually ruled about a hundred years earlier than this novel is purportedly set. In fact, the book is set no more than ten years before the entire Khazar state was destroyed by Sviatoslav of the Rus.) His characterization of Christians, Muslims, and the Rus themselves also border on uncomfortable at times, although they fall shy of being completely insulting or overtly offensive, and from the point of view of his characters is probably reasonably historically accurate anyway.
The biggest problem I had with it was that for a swashbuckling romance, it was surprisingly somewhat light on the swashbuckling action. Chabon obviously liked his characters, and made a point of making them interesting and intriguing to read about, but the plot was more of a talking heads plot than I would have expected going into it. I suppose that's not unsurprising, given a genre novel written by a "literary" writer.
My end result, I suppose, is that the novel was interesting, quick and dirty and easy to read, and a great idea with great characters, but somehow less than the sum of all its parts, at the end of the day. I'd recommend it, but if it were much longer than it is, I'd do so much more hesitantly.