I'm old enough to remember when it was raised here in California as a legal cash crop. It was raised for fiber to make rope. I recall being with my Dad and having him note the distinctive smell, he would say: "What's that guy smoking? A rope?"
Even though I am anti-weed, I cannot say that it will cause Gynecomastia any more than it will fit other of the long list of false things said about it over the years by well meaning but misguided people.
The first truth about Marijuana is that no one is telling the whole truth. The Anti-Marijuana people are still telling exagerated stories about how evil it is and the Potheads are still saying that it has no effect on them. Both are lying and we know it.
The relationship between Pot and Gynecomastia is not an easy case to make, either for or against, if you are talking to a devout user or equally dedicated non-user. We are well aware that there are some regular smokers who do not have Gynecomastia. Yes, we have heard all the arguments both ways.
There remains one fact that cannot be denied, Marijuana users have a higher incidence of Gynecomastia than non-users. Given the fact that users often try to conceal their habit, the relationship could be more significant than we might expect..