I'm not entirely sure, but although we don't know exactly what Arnold took back in the 60's, it sure wasn't "mega dosages" compared to the bodybuilders of today - if you look at other guys of that time, like Sergio or Lou Ferrigno, they never had gyno either. But again, it may have just been genetics. Franco had it a bit later on towards the late 70's.
Some guys have gyno even before touching a weight - it's just the way that the hormones in their bodies work during puberty - after they start lifting it can get worse and once they start using gear it gets worse still. Once you get to that point it's irreversible and only surgery can get rid of it.
steroids back then were, legal, cheap and readily available....
sure arnold wasn´t megadosing

the were mega,mega dosing!!
only thing that was still very very rare was hgh. and they were not using slin yet.
why some do not get gyno? GENETICS!
i can go as high as 1200 mg test, my nipples do not even itch!
and you have guys when they so much as look at an ampoule durateston, they get titties!