I think what it meant was it is the people/drug dealers who make it a gateway drug. You're with a weed dealer and he tries to sell you coke, thats not the weed doing this, its prohibition. Marijuana is only a "gateway drug" because you are forced into the black market where other harder drugs are also sold. It could also be meaning that you smoke weed with your friends and this connection to the underworld and the relationships with the people there could lead you to other drugs.