imagine a scenario, coffee is illegal. you can't buy it in a normal shop, but the same guy that sells every other illegal substance has it. and you have to go to that guy to buy some coffee

even though coffee is not similar in its effect to any harder drug by going out to buy it you're gaining the skill of obtaining illegal substances, and also put yourself in a place where you can get a variety of them

as you're buying your coffee your drug dealer tells you "try this, it'll make you feel really good" and you know what? society's already lied to you about coffee being bad, maybe they're lying about the ecstasy thing too, you already trust this guy, he's been selling you good coffee, so might as well try something new and exciting too eh?

and to be even more blunt, what i'm trying to say is that it's not the substance itself that makes a drug be a "gateway" it's the way you obtain it

if weed was perfectly legal and sold freely then it would not longer be a gateway drug, because as you try it you don't gain the experience and skill of obtaining illegal substances, nor do you get placed in a position where you can grab something extra with your purchase