Marijuana wouldn't be a gateway drug if it was legal. The government used the argument that marijuana led to harder drugs back in the sixties, namely that it leads to deadly heroin. In the eighties and nineties the big scare drug was crack. As of lately the partnership for a drug-free America has been shying away from the more outlandish claims of insanity, rape compulsion, violence and death and manly focusing on the more believable, but still not entirely true, allegation that marijuana will turn you into a boring zombie. However, the "gateway drug" theory is only valid for marijuana when a person must go to the black market to get it. "Gateway drug" is not a phenomenon based on a need to increase the high produced by a drug. It is a factor of availability among social groups. When you make friends with one person who smokes pot, you'll meet nine more. Of those ten people, maybe one of those people is, or has a friend who, uses and can get cocaine. If, through your choice, you happen to pursue the opportunity to try this drug, then you have passed through the 'gateway' to a second tier of social collectivism; you are now amongst cocaine users. Amongst these new acquaintances, you may meet someone who can get heroin. And the process repeats itself.
Now imagine that pot can be bought at your local supermarket, provided you show ID that shows you're over 18. Suddenly you don't have to be in a social loop to get the stuff, and as a result, you are by default not made privy to the availability of other illegal drugs.