g-man is gordon freeman
What the fuck are you smoking?
The identity of G-man is mysterious as hell. No one is quite sure about him. I, myself, think he may well be some kind of God-like figure.
He is invincible, and somehow gets Gordon out of that explosion (otherwise there'd be no HL3), which suggests that he controls Gordon's fate. 'The "time" has come again,' I recall Gman saying at the end. Time for Gman to rescue Gordon from death? The time of Gordon completing his assignment again? The story kind of revolves around the mystery of Gman.