Pot was the 1st drug I used as a teen, then I went on to drinking. All my friends used it too. Most went on to using other drugs and left pot far behind. When I had children I stopped smoking it and stopped drinking. However many people I know swear its a gateway drug. I doubt it will be made legal as too much money is made from it being illegal. I feel bad for the people who want to use it medically.
On the upside, I had the best night's sleep ever when I went to bed afterwards.
"Gateway drug" is a term many different meanings as to render it virtually meaningless.
As far as I know there is nothing about marijuana that specifically makes you more likely to want to or use other, "harder" drugs. For people who seek new sensations and the thrill of the forbidden (as young adults often do) pot is naturally the first illegal drug they encounter, being the most prevalent in most areas.
From there, I'd say two factors come into play. One is that getting involved in using MJ regularly is likely to put you in contact with use, sell or can find you other drugs. Second, having experimented with pot and not found it to be the terrible bogeyman that the authorities have made it out to be, one is likely to discount the other (often valid) warnings that they have heard.
My personal opinion is that we'd do a lot better trying to educate people to respect their minds and bodies and use substances responsibly. Look at booze, for example. A drink now and then, or even an occasional bender, is not necessarily a bad thing. But drinking and driving, or becoming an alcoholic is. For whatever reasons, though, and I can think of a few, many young people use alcohol completely irresponsibly. Similarly with pot. Coke can be used occasionally and responsibly. Not sure about heroin - certainly most people have to desire to stick a needle in their arm just for fun.
Anyway, I think prohibitions of various substances miss the point: Why do so many people wish to lose themselves in a drug-induced state for so muc of their time? We don't seem to have put very much time or energy into answering that question.