This study analyzes the effects of the gender (employee-) tenure gap (GTG) on firm value and stock returns. We first find that firms with a low GTG have higher profitability and firm value than firms with a high GTG, as the low GTG yields competitive advantages. We further validate the low GTG’s competitive advantage by using an event analysis based on Korea’s military service period changes which affect the male labor forces. We show that low-GTG firms earn higher risk-adjusted returns than high-GTG firms, consistent with the argument that the market is not fully aware of the benefits of the advantage from a low GTG.