Given the way he talks of Freeman as an "item" or "investment" or of Alyx as a "long shot business gamble" i am not so much a supporter of g-man is freeman thing.
He seems more like a middle man, like a weapons dealer that double deals under the table, or something. Only for more powerful races and throughout the galaxy/univereses.
He also seems to possess certain psychic powers that enable him to slow down time, transmografy, telekinesis, and show up "in your mind". None of which are human traits, even if he is a future self it's unlikely he would be so detatched from his past self.
He also discusses certain business rules such as not directly intervening in events. Plus a time paradox of Grodon saving himself in the past from the future is kind of a plot hole. Oh yea, and in HL1 he has green blood. Why would he have green blood if he was human?