For that matter you could also call Cigarettes or Coffee a gateway drug.
How many weed smokers do you know that don't also smoke cigarettes? Or haven't ever had a cup of coffee? The correlations are loose IMO. Yes, a lot of end users of the higher-end drugs like Crack, heroin, etc smoked marijuana before they got addicted to those hard drugs, but they probably also smoked Cigarettes and/or drank booze. Are those gateway drugs too? The only reason Weed is seen as a "gateway drug" and those aren't, is because the other 2 are legal. To get weed, you have to go to a drug dealer, who often supplies multiple drugs. Once you get involved with characters that are unsavory and nefarious, lots of bad things can happen. I am not saying all drug dealers are bad, or that all drug culture is bad, but the whole gateway drug thing is overblown. It is more of a gateway to the drug culture, which will then lead to harder drugs if you choose to pursue them.
If weed was legal, it would not be a "gateway drug."