The story
The source of the legend of Gambrinus is uncertain and perhaps based on a mythical Germanic king Gambrivius. The etymology is unknown, but many explanations exist: * a corruption of Jean Primus, John I, Duke of Brabant (1252–1294) * Middle High German gambra (“germination of grain”) * cambier, name given to brewers in the north of France in the Middle Ages
Etymology from Wiktionary, CC BY-SA 4.0.