Who is the G-Man, and what is Aperture Science doing with Black Mesa?

Episode 2, before death to Eli Vance. Father of Alyx Vance.
Dr.Kliener, Alyx, Gordon, and Eli was looking on how Judith Mossman was doing. Until the combine came, and was going to capture her, she also had a footage of the rare, undiscoverd, Borialis.
A lost ship set from Black Mesa. When Kliener is talking about the boat, he mentioned.. 'Aperture Science.' Also on the pictures, scouting the Borialis shows a map, on the top left saying.. "Aperture Science" and on this ship was also a sign of Aperture Science. While thats going through, earlyer in the Episode 2. G-Man mentions, "Unforseen consequences" to Alyx, while Voritgaunts are meditating to heal her. But why telling her? Maybe because Eli knows who G-man is, obviously, Dr. Wallace Breen knows who is G-Man. Maybe Cabbage also. (Cabbage is the Rebel leader in Half Life 2, witch shows you how to use an RPG) Who IS the G-Man? Give me your opinions and theories. Ill see witch is the most knowledged comment, and reply to your review.