A quantity of marijuana or hashish, ostensibly worth $50, but which has been taxed. By "taxed" is meant that the drug has been procured through a middle-man, who has taken some of the product for himself, secretly (or, to use the vernacular, "on the sly"), as covert payment for his services. Whilst taxing is standard procedure among Marijuana Addicts, the Gay Man's Fifty is distinctive because it is taxed excessively: the marijuana is so heavily taxed that it is thought (by the middle-man) that the intended recipient will not accept it. It being a seller's market, fortunately for the middle-man, the recipient always does.
"Yo, I thought that dude was my cuz, but he sold me a gay man's fifty, son!"