The gateway theory, as it relates to psychadelics and other arguably not-so-harmful drugs, is an incorrect concept because 1. these things are not all necessarily all that bad for you, 2. they are a in fact a good thing to experience in many ways IMO, 3. if you try pot, you usually have an eagerness to try these things in the first place, the reason you are trying them is not because pot made you; pot only reassured you. The true desire came from within and no buzzword or law enforcement policy is going to be able to tackle that.
The gateway theory, as it pertains to drug ABUSE, is also a bad concept because here, again, the drive comes from within. People who go on to harder drugs do so because there is something in their lives which makes them not enjoy sober reality. They are perhaps just bored, or perhaps they have deep emotional and/or mental issues (TONS of people do). They do not go on to other drugs because of pot, pot is not the driver, it is the vehicle. You can't stop a pissed off driver even by taking away their car; they will find another car. These people need to feel good, and pot does that to a young person with no past drug experience. Many people find it unsatisfactory after a certain amount of time and move on to whatever does the job.
Pot is not the problem, people are the problem, and so long as humans exist on this earth, there will be people with problems, and those people will seek mind-altering substances or activities.