Marijuana is not a gateway drug. It does not lead to a desire to do anything other than eat, sleep, and smoke more weed.
The gateway part is the lifestyle that develops when doing it regularly. The people you have to hang out with that will tolerate your being high in strange or inappropriate times or situations become less and less wholesome. The people you have to go to for a more reliable and better quality supply of cheeba for the best price become more and more shady. And most importantly, the places you and your stoner friends congregate attract more attention from the smart, savvy, crack and coke peddling entrepreneurs who know a patsy when they see one. And in the state of extreme paranoia and/or lax inhibitions and sense, the idea of other drugs either seem like the best thing to do to make you look and feel more alert when you feel like you are being watched or followed or attacked by the cops or parents or whomever, or they just sound like they'd be fun and cool and better than the weed you are too used to and too immune to, to the point where it is costing you too much to get just a little high.
Without these dirty mother fuckers circling around looking for poor potheads like you to pounce on when you are in the worst possible state to defend yourself, coupled with a moderate dash of self control, there would be no threat of drug escalation. All you'd have to worry about is where to get the cupcakes and who ate all the peanut butter. Abuse or over use of anything will lead to a downward spiral into depression, debt, alcohol, drugs, or whatever seems like the best and easiest option at the time. Food, videogames, booze, even comfort-shopping can be an addiction and can lead even the most well-meaning people down paths they'd never go if they were in their right minds.