Dialect is completely irrelavent to determining someone's sexuality. Any examples of him showing attraction to other males in the series? If not then I wouldn't say Gumball is gay. However, Gumball is based on Princess Bubblegum, and she is bisexual, so I would assume Gumball is bi because he is based on her.

I would defiantly say Gumball is not straight. I could see him having small love affairs with Marshall Lee. During 5 Short Tables they said regrets and Marshall showed up and of course that's a back story but I imagined them having a big break up fight. But, just because a LGBTQ+ person is doing the voice of the character does not make the character LGBTQ+