Indian stock markets closed with modest gains on Sensex monthly expiry day, as both indices erased significant intraday advances. Sensex added 109 points to 77,100.47, and Nifty 50 climbed 34 points to 24,056. Strong performances from IndiGo, M&M, and Maruti Suzuki bolstered the gains, while Power Grid shares saw notable declines. The broader market, however, dipped into the red.