He shouldn't have been gambling in the stock market, and ended up losing heavily.

gambling 是动词 gamble 的现在分词形式。gamble 的意思是"赌博、冒险"，指在不确定结果的情况下投入金钱或资源的行为。在股市语境中，gamble 特指"投机、冒险投资"的行为。