If you're looking for anime similar to Gintama, you might like these titles.
Ten-year-old genius Chiyo, animal-loving Sakaki, loudmouth Tomo, athletic Kagura, weight-conscious Yomi and dim-witted Osaka are six friends who share laughs, good times, and a high school homeroom. With scary (and sometimes perverted) teachers, school festivals, penguin suits and general hilarity abounding, you can be sure that there's never a dull day in the life of one of these students!
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Their premise is not at all similar, but both have a large cast of strange and eclectic characters. Both animes use absurdist humour and parody.
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Although their premises aren't too similar, the characters Gojo and Gintoki are soooo similar. They're both white/silver haired mentors to young teens who are incredibly strong, who grew up with a dark haired best friend who went crazy and betrayed them. Both Gin and Gojo are outwardly fun loving and ridiculous men in their late 20s who actually have a dark inner world they're trying to hide or run away from.