It was a decision decided by the mob as a whole, more or less, and not just Gambino that narcotics were prohibited. Of course, history has shown that there was plenty of mob involvement in the drug trade before and after 1957. Even the bosses who were supposedly tough on drug involvement (Gambino, Corallo, Accardo, Castellano, Gigante, etc.) had subordinates who were involved in it. Some of which ended up reaching high level positions in their crime families. Besides "Don't get caught," the unofficial rule seems to have been, "Don't deal unless you have permission."