A “gateway” is an opening in a gate that lets you walk from one side of the gate to the other. You probably knew that already, but hold the image in your mind.
A “gateway drug” is a drug you take (for fun — not because you need it) that supposedly leads you to take other, more dangerous drugs.
The idea is that if you use a “gateway drug,” you are walking through a gate into a place you couldn’t get to before — a place where people use drugs for fun. Once you step through the gate, you’re just as likely to try a super dangerous drug as to keep using the relatively harmless one that opened the gate for you.
You can see the problem: Who says you can’t simply turn around and walk back through the gate? Or hang out just inside the gate without ever feeling tempted to wander over to the “drugs that might kill you and will definitely at least ruin your life” area?
In this sense, a gateway drug is just a slippery slope in disguise.
And you’ve probably noticed that fun (and sometimes dangerous) drugs like alcohol and caffeine are used by huge numbers of people on the “we would never consider using drugs for fun” side of the gate.
So: Why do some of us tell this story about the gate? What is appealing about the idea that people who use certain drugs for certain reasons are different and separate from everyone else?