Gordon is described by the events and other characters in the Half-life series. He's not some narcissistic window for you. He has a personality, and if you pay enough attention to the dialogue of the other characters while he's around, as well as the actions that the plot forces him to take (it's a linear story people, you can't actually change what he does very much) they very fully describe what he is like.
IMO Gordon's silence was purposeful so that you wouldn't be jarred out of his (VERY WELL DEFINED) character by some voice you didn't like. See: Batman, Watchmen for oft complained about voices that ruin characters for some people.