Gordon Freeman is the protagonist of the Half-Life series, a famous FPS franchise developed by Valve Corporation. Valve didn't include any lines for Gordon Freeman, so that it would be easier for a player to immerse themselves in the game.
His quietness is not only just something set by Valve. In Half-Life 2, a character named Alyx Vance humorously jokes about Gordon's silence, saying:
Man of few words, aren't you?