Not for me. I became a huge pothead in late high school/early college, and it never made me want to go near any other drugs (I was already drinking, but that started earlier). I later began experimenting with mushrooms, but I wasn't even smoking pot at the time so there is really no correlation between the two for me. In fact, even when some of my friends were into the idea of hallucinogens in my pothead days, I was too freaked out to want to try them until I was way more stable of a person.
I'm not saying it never happens, but one of my major issues with the "gateway drug" thing is that it's blaming a substance, which does not have any say in these matters, for the curiosity and/or stupidity of humans. And it doesn't give PEOPLE enough credit.
Compared to most other drugs (including alcohol) pot is a harmless substance (with a few exceptions) that mostly just chills people out. This detail is significant because the average pothead, from my experience, is not going to say "I really like pot... maybe I should try some cocaine, which is going to have a totally different effect on my body and possibly kill me if I do too much." Do you see what I'm saying? It's not a logical progression at all. It's not like "Hey, you like pot? Then you'll REALLY like this meth right here." They have nothing in common- one is incredibly horrible and dangerous and can make you a complete monster, and one is going to make you pass out on the couch with a bag of Cheetos next to you.
I can't speak for the education system/parents of kids all over the place, but I was educated enough on drugs to know that some of them are just bad fucking news, and smoking pot (as much as I think it was a bad thing for me at that time in my life) did not change this knowledge or my common sense. I was offered cocaine on a few occasions in my teen years (mostly by people who were older, for the record) and I never touched it because I knew better.
Yeah, of course it's possible that smoking pot can give you easier access to other stuff, depending on what kind of company you're keeping, but nobody can open that "gate" except you.
edit: sounds like some of the replies that showed up while I was writing this novel, made a lot of the same points, wooo!
I definitely agree with the addictive personality thing, too. I do not have one of those.