In January 2021, GameStop experienced an extraordinary surge in its stock price, soaring from $17.25 on January 4 to a pre-market value of $514.50 on January 28. In contrast to previous studies, we use tick-by-tick data of stock and options trading to demonstrate that this remarkable surge come mainly from overnight trading, driven mainly by institutional orders rather than those from retail investors. Although sophisticated option traders typically maintain a positive gamma position, option market makers skillfully regulate their gamma exposure by participating in retail option trades. However, an “after-hours gamma squeeze” was initiated by a twitter catalyst, causing eventually the well-known GameStop short squeeze. We also provide an extended model of Brunnermeier and Pedersen (2005) that explains some of our major findings.