The only reason why weed is a gateway drug is because you have to go to a drug dealer to get it. More than likely the person who you are getting your weed off is going to be selling something else which will then become available to you. If you didn't have to go to a drug dealer to get weed then you wouldn't come in contact with other harder drugs. Most people sometime in their life will want to smoke weed and will go looking for it, however there a very few people that will wake up some morning and say "Yeah, I'm going to go and do heroine tonight." It is when the person that they have to go to to get their weed who is also selling harder drugs that people become tempted. I can't see weed being legalised anytime in the near future but it has to be decriminalised to the point where harder drugs will not become mixed up in it, like how they have it in Amsterdam.