What is the connotation for gay蜜 and 闺蜜?
gay蜜 is not a formal word, usually used on Internet or in oral languages by the young.

When a girl has a gay male close friend, she will call him "gay蜜".

e.g. "他是我的gay蜜。" means he's my close friend and he's gay.

Is "gay蜜""闺蜜" usually used for young adults (18-30 years old)? Yes.