G-Man is a very mysterious man with very remarkable powers; he's employed by an unrevealed party, apparently with the task of recruiting and managing other, useful lifeforms for an unknown purpose (and possibly for the highest bidder).
He's been Gordon's employer (since the end of Half-Life), but in Episode One, it is said that he will lose control.
Dr. Gordon Freeman in the flesh, or rather in the Hazard Suit, I took the liberty of relieving you of your weapons; most of them were government property. As for the suit, I think you've earned it. The border world Xen is in our control, for the time being, thanks to you. Quite a nasty piece of work you managed over there. I am impressed. That's why I'm here, Mr. Freeman. I have recommended your services to my employers, and they have authorized me to offer you a job. They agree with me that you have limitless potential.
You've proved yourself a decisive man so I don't expect you'll have any trouble deciding what to do. If you're interested, just step into the portal and I will take that as a yes. Otherwise ...well... I can offer you a battle you have no chance of winning. Rather an anticlimax after what you've just survived.
Well... it looks like we won't be working together; no regrets, Mr. Freeman, but there are a few survivors of your personal holocaust who would like the chance to meet the man responsible for the total annihilation of their race.
Rise and shine, Mr. Freeman. Rise and shine. Not that I wish to imply you have been sleeping on the job. No one is more deserving of a rest. And all the effort in the world would have gone to waste until…well, let's just say your hour has come again. The right man in the wrong place can make all the difference in the world. So, wake up, Mr. Freeman. Wake up and smell the ashes.
Cleverly done, Mr. Freeman, but you're not supposed to be here. As a matter of fact, you're not. Get back where you belong, and forget about all this. Until we meet again
Time, Dr. Freeman?
Is it really that time again? It seems as if you only just arrived. You've done a great deal in a small timespan. You've done so well, in fact, that I've received some interesting offers for your services. Ordinarily, I wouldn't contemplate them. But these are extraordinary times. Rather than offer you the illusion of free choice, I will take the liberty of choosing for you...if and when your time comes round again. I do apologize for what must seem to you an arbitrary imposition, Dr. Freeman. I trust it will all make sense to you in the course of...well... I'm really not at liberty to say. In the meantime... This is where I get off. Until we meet again