Is Gintama: Dai Hanseikai hentai?
No
Rated PG-13 (Teens 13 or older) on MyAnimeList and not tagged as Hentai.

Special · 2010 · 1 episodes · PG-13 - Teens 13 or older
Action, Comedy, Parody, Samurai, Shounen
Some of the characters get together and talk about "regrets" they have after 4 years of anime Gintama. Soon they fight over who gets more screen time. Special animation shown at the Gintama Haru Matsuri 2010 live event.
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