Used attributively before another noun (e.g., "gateway drug", "gateway city") to describe something that serves as an entry or means of access.
introductifAu sens figuré : qui ouvre la voie à autre chose, qui sert d’initiation. Many people worry that vaping could be a gateway habit leading to cigarette smoking.
Beaucoup de personnes craignent que le vapotage soit une habitude introductive menant au tabagisme.