Also, I think a lot of the scenes where someone is talking at you don't really make much sense if Gordon was just standing there stonefaced. He'd look like a complete sociopath.
Originally he was clearly just a silent protagonist which became a little weird in HL2 because there was much more interaction with NPCs. Valve then played off that when Alyx says stuff like "Man of few words, aren't you?".
I wouldn't be a huge retcon but it's pretty clear he wasn't supposed to be a mute from the start. It's kind of like saying Gordon has telekinetic powers because he can control the steering wheel of his car without using his hands.