The G-Man

Reminds me of a theory I heard once. It was that the G-Man didn't actually exist, and that he was just someone Gordon invented in his mind, to cope with the whole "Alien Invasion, life's work ruined" thing. Him showing up beforehand is just him sticking the G-Man in later. Heck, he could've met Shephard later and told him, and Shephard's brain put him in too, to explain doors opening, the bomb being rearmed, etc. Think about it.