A alcoholic beverage with a low alcohol content consumed innocently without the intent of getting buzzed or drunk. It ends up being the starting point before moving onto hard liquor and the inevitable hangover that will follow.
“Last night I was out for sushi and drank some sake. At some point I moved onto high end vodka and now it feels like a gang of honey badgers had an orgy in my skull before parking a van on it.”
Examples of Gateway Drunk beverages include, but are not limited to: Beer (non IPA), wine, sake, Mike’s Hard Lemonade (and similar products).