When I was younger, although smoking pot, was strongly opposed to amphetamines and it took many years before I tried pills - and that was a big effort in itself.
I think the gateway tag comes from timing and suitability. It's generally the most accessible drug for young people, the most social in acceptance and the act of sharing a smoke, and most suitable for social situations at a younger age.
Most people cant handle, or don't want to be stoned when they are out in a nightclub, so they move to amphetamines. I don't know anyone else who can deal with a dance club when they are stoned.
For a couple of years, I hardly drank. Took pills most weekends, and was far better behaved, spent less money, had a great time, connected with people like never before, experienced music like never before. We had a much better experience on pills than alcohol - for a while. Gets to be too much for your brain after a bit.
/end jabbering on about shit.