Weed can definitely be a gateway drug. Problem is, so can every other substance we know of. The focus is on cannabis as "THE" gateway drug because it's so prevalent, so common even amongst the youngest, most vulnerable members of our society.
Honestly, weed is definitely A gateway drug. I mean, I've seen on this thread people saying that using marijuana makes you more open-minded to trying other illegal substances and then they go on to say that it's not a gateway drug. (Side-note: What do gates do? They open and give you access to areas previously inaccessible. There's a connection here.) A positive experience with marijuana is likely to diminish many younger peoples reservations about trying other drugs under logic similar to "Well, they said it would turn me insane after a hit, I just smoked a blunt and feel GREAT. I bet they lied to us about [insert random substance here] too, maybe that feels even better?"
I just feel like marijuana and every other drug can be coined a "gateway drug," I just don't view that with a negative connotation. I'm all for responsible drug use so I truthfully don't view the willingness to educate yourself and try other psychoactive substances you may be interested in as a bad thing. At the very least it's a neutral situation in my mind. The scale doesn't tip one way or another until a person loses sight of the whole responsible drug use thing (which I admit can and does happen).