In my past experience, marijuana isn't a gateway to more addictive drugs per say, at least no more then alcohol or tobacco in fostering addictive behavior. But what it does so is introduce buyers into the underground/black market (which ever you'd prefer to call it), because the only way for a majority of people to acquire it is illegally. And once you are in contact with that market, it opens the door to other illicit substances such as heroin, cocaine and methamphetamines. So in that sense it generally acts as a gateway, but it is by no means a definitive precursor to harder addictive drugs. Legalizing would help to eliminate this problem. But that isn't all that need to be considered before it were to happen. There are a ton of other factors legalization would have an effect on.