An alcoholic beverage one consumes prior to becoming drunk. A gateway beverage can be; beer, a Mike's Hard lemonade, a fruity beverage, Smirnoff Ice, etc . Consuming multiple gateway beverages "opens the gates" to the possibly of getting drunk or tipsy but does not necessarily mean you are getting drunk... just enjoying a couple beverages.
Gateway beverages include; Mike's Hard Lemonade, Smirnoff, Malt Beverage, Beer, Fruity Drink, Something cold blended with alcohol.
Friday night you come home and have a few Gateway Beverages with friends. Or Saturday afternoon by the pool a group of friends can have Gateway Beverages together.