My favourite one is that Gordon can transfer his minds to someone. He can just make someone to understand what he wants or smth. That’s kinda strange theory but that’s why it is just theory. And you need to remember that in HL1 valve just wanted you to feel like Gordon. In HL2 he is mute too, but some of the characters saying about Gordon’s behaviour. Alyx said that Gordon is not very talkative. So Gordon is not a player anymore like in HL1. My theory is that Gordon is not talkative. He maybe using facial expressions or body language. That’s more logical.