Is Cardfight!! Vanguard G hentai?
No
Rated PG-13 (Teens 13 or older) on MyAnimeList and not tagged as Hentai.

TV · 2014 · 48 episodes · PG-13 - Teens 13 or older
Action, Strategy Game
Taking place after a three-year time skip, the series will follow Shindou Kurono, Kiba Shion, and Anjou Tokoha as the new protagonists of G. Kurono uses the never-before-seen clan Gear Chronicle, which brings together the destinies of these three toward an "impossible future, an impossible self signifying 'Generation.'" (Source: vanguardus.blogspot.com, edited)
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