When you read a comic, you take two sequential panels and your head fills in whatever lies between them, like an action, a passing of time or a change of location. If in the first panel you see a man and a pile of boxes, and in the next the boxes are piled in the back of a truck and the man is holding a cloth to his brow, the panel combination causes you to "see" him loading up the boxes, sweating and wiping the sweat off.
From the reactions of other characters it seems obvious to me that "Gordon Freeman" actually does talk, but it happens between panels, so to speak. Your imagination fills in as much detail as necessary. Freeman isn't a character as much as a label and an empty vessel for you to fill. In a voice acted game, not hearing your own character's voice is an artistic choice that makes it easier. Erasing the text is just one step further.
A fleshed out character that was genuinely mute would be totally different.