Is Batman: Gotham Knight hentai?
No
Rated R (17+ (violence & profanity)) on MyAnimeList and not tagged as Hentai.

OVA · 2008 · 6 episodes · R - 17+ (violence & profanity)
Action, Adventure, Martial Arts
Anime-inspired direct-to-DVD anthology film. Comprised of six short stories, from diverse creators, including Academy Award-nominated Josh Olsen (A History of Violence), Batman Begins writer David S. Goyer, and comics scribe Brian Azzarello. It's planned for a release window of two to four weeks prior to the release of The Dark Knight, and would bridge the gap between Batman Begins and The Dark Knight. (Source: IMDB)
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