If there is a "gateway" effect, it was created by prohibition.
When kids are subjected to the DARE program, marijuana is lumped together with all the other, more dangerous drugs. When they experiment with marijuana, and find out that it doesn't turn your brain into a fried egg, they wonder what else they've been lied to about...
Is alcohol not a gateway drug?
Lastly, just because a troubled person uses marijuana, it doesn't mean that marijuana is the cause of the troubles. People with addictive personalities will always find something. Yes, Pot can be addictive. Just like shopping, gambling, or sex.