Gateway theory is a hypothesis. A very criticised one though for the reasons (pulled off wiki) below.
Some individuals are, for whatever reason, willing to try any substance, and the "gateway" drugs are merely the ones that are (usually) available at an earlier age than the harder drugs.
Particularly for cannabis, which is illegal, exposure to the black market (where harder drugs are available) is suggested to be the real cause
For teenagers, credibility of adults is eroded when the dangers of the "gateway" drugs are exaggerated or made up, leading them to think all anti-drug messages are nonsense.
The peer environments in which "gateway" drugs are used can sometimes overlap with the ones in which harder drugs are used, especially in societies that prohibit the substances or impose very high age limits.
Due to the nature of their merchandise, dealers of the harder illicit drugs may be unwilling to reveal themselves to those unfamiliar with the "gateway" drugs.