and what does gay mean ? It used to mean 'very happy', but now it almost always means 'homosexual'. This use of the term 'gay' is still generally considered to be slang. eg
There's probably some history and some evolution to it, but "a guy" has always been a male person. I think in terms of the 40's musical, Guys and Dolls, where guys were the boys and dolls were the girls. I believe in those days they were thought of as "eligible" and sort of interested in each other as "opposite ***." These days we've added a usage where we can address a mixed group as "guys."