Is Overman King Gainer hentai?
No
Rated PG-13 (Teens 13 or older) on MyAnimeList and not tagged as Hentai.

TV · 2002 · 26 episodes · PG-13 - Teens 13 or older
Adventure, Mecha, Sci-Fi
Sometime in the distant future, mankind is forced to live in domed cities while the rest of the world is left to recover from years of environmental hardships. In the Siberian Dome-polis, professional gamer Gainer Sanga and mercenary Gain Bijou commandeer an Overman and lead a mass exodus out of the city. (Source: ANN)
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