“Gamzee appears to be the troll equivalent of a Juggalo, belonging to a cult of capricious minstrels, and his text is littered with references to Juggalo subculture. So much so, in fact, that it's something of a miracle in itself that he hasn't uttered the phrase "fuckin' magnets" yet (in a rap-off with Tavros, he paraphrases a complete stanza of ICP's hit single 'Miracles'). Another possible reference to his design as a Juggalo could be his strife specibi, which is clubkind jokerkind. Of these, he uses juggling clubs, a pun possibly on Juggalos. Though he swears quite a bit, he is a very good natured and laid back troll. He is without question a supportive friend and will do things and make sacrifices, even stop eating sopor-slime pies and drinking his wicked elixir, Faygo, at the request of a friend. Because of his magnanimous personality, Gamzee will frequently act as an advisor or even a therapist to his friends, such as Karkat and Eridan, if they are experiencing emotional stress, though his usual counseling involves them waiting for a miracle.”

(Taken from the Homestuck Wiki)

Honestly, I’m thinking of more of a raspy stoned voice for him. He would have a medium-low pitch/tone in his voice.