I believe G-Man and other "people" are just god-like people, playing a game. When Dr. Breen mentions that Gordon was a high bidder, what do you think he was talking about? I believe it is somewhat like a game, where they get some what of heroic humans such as Gordon Freeman and Adrian Shephard (wikipedia him up), and put them in situations like Black Mesa or City 17. If they accomplish to do what are told or what comes towards them, the man who bidded on him to win, such as G-Man, wins money or more power.
Eh, it's not very detailed, but it's one of my theorys.
Eh, it's not very detailed, but it's one of my theorys.