I plowed trough Half Life 2 recently. In the end of the game Dr.Breen is talking with the Combine advisors and says something about a host body. What if Gman is a Combine advisor who has possessed a human and is now helping Freeman trough to defeat his peers for whatever reason? It would explain the teleporting around and strange time stopping behavior.
Ehhhhh
I find it highly unlikely
How would a combine advisor possess a human? All they can do with them is feed off them and suck out information from them. (They can most likely do a lot more than that, but I doubt they can possess a human.)

I really want to find the full image of this, I remember it being bigger and there was a cat with a tinfoil hat on sitting on the table by him.