If you were looking for the "Leavers" content (graduation) for Gossip Girl, this is the definitive episode.
The group heads off to college—Serena to Brown (eventually), Blair and Dan to NYU, and Nate to Columbia. [S2E23] The Leavers Dance (2)
This episode serves as the Season 2 finale and is famous for its cliffhangers and emotional transitions. If you were looking for the "Leavers" content
Blair officially passes her crown to Jenny Humphrey , marking the end of her reign at Constance Billiard. Blair and Dan to NYU
After a season of "will they/won't they," Chuck finally tells Blair "I love you" and they officially become a couple.