I think an assumed conversation would be even more annoying.
The whole point of HL2 is Gordon being 100% under your control. If Gordon interacted with the characters in ways you didn't set up, you'd lose that.
Even if you don't hear him, you'd still have times when you think "what kind of idiot would say that?"
Gordon doesn't say anything. He's tall, nerdy and mysterious. And probably a bit of a headcase. Everyone knows this. They just accept it. He's seen some crazy things, and just doesn't feel like talking about it.
Making him talk, or worse having assumed conversations that, for some reason you can't hear, would be annoying and weird.
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