First off I think we should define what a gateway drugs is. Is it that pure use of the substance causes you to use other drugs, or the situations surrounding the use of the substance causes you to use other drugs?
If the former, then no, definitely not. The idea that taking one drug can make you use another, completely unrelated drugs is preposterous at best. No one has ever taken their first hit of weed and gone "Wow, I could really use a shot of H with this!" (Well I'm sure someone prolly has, but s/he was prolly a junkie to begin with).
If the latter, then yes, it would be, simply because marijuana is an extremely popular, easily obtainable illegal drug. Being around or obtaining ganja generally means being around people who either use or sell other drugs as well. Because of this you're more likely to try harder drugs than someone who never smoked weed before.
Also, anyone who suggests that alcohol, nicotine, caffeine, or any other legal drug is a gateway drugs is fooling themselves. I've never bought an energy drink or cigarettes and had the cashier offer me a little cocaine to go with it.