In what Half-Life expansion can you find Gordon's picture in an "Employee of the Month" picture frame?

In the Half-Life expansion, Opposing Force, players can find Gordon's picture in an "Employee of the Month" picture frame. This is because Opposing Force focuses on the events of the Black Mesa Incident from the perspective of Corporal Adrian Shephard, who encounters and interacts with Gordon Freeman throughout the game. The inclusion of Gordon's picture in the picture frame is a nod to his significant role in the Half-Life series and serves as a reference for players familiar with the main game.