It really isn't hard to disprove the gateway theory. First the number of Marijuana(MJ) users is an order of magnitude greater than users of hard drugs. Its gateway effect must be pretty weak. Secondly, the Institute of Medicine report of March 1999 that determined that there were medical uses for Marijuana also determined, from a study of the literature, that the only characteristic of MJ that seemed to support the gateway theory was its legal status. That is the fact that you had to go to the black market to buy MJ created your connection to the black market for the hard drugs.
I will add to this the propensity for the government to lie grossly on the health effects of MJ, lies which are obvious to our kids, encourage them to try the harder stuff since they lose all faith in the truthfulness of the govt.
Conclusion: Keeping MJ illegal encourages the use of hard drugs.
What do you suppose criminalizing marijuana use does to encourage marijuana use? Would consumption go up or down if it were legalized? Has it? What is the observed effect on hard drug use?