The question is whether Marijuana use in and of itself encourages or preface additional drug use.
I would argue that in many ways it does. Marijuana is–or was–illegal. Alcohol is legal, but age restricted. If you are willing to use a substance that is (was) entirely illegal, you are more likely going to be willing to try other drugs that are legitimately addictive, because you’ve already crossed one of the major hurdles. If alcohol had been illegal for the same amount of time that marijuana had been, then I would agree that alcohol was likely a gateway drug as well.
I’m in favor of de-scheduling marijuana entirely. But I think that it’s disingenuous for people to act as though there weren’t serious problems with chronic and underage marijuana use.