Actually, I don't think there's a direct connection between the G-man and the Administrator. The Administrator was some 'higher up' dude at Black Mesa (maybe the director of the reseach facility), judging from the dialogue in the Test Chamber. The only possible connection to the G-man is that whoever G-man is working for they seemed to have pull a lot of the strings behind the scenes at Black Mesa. So in that sense, G-man ownz the Administrator.
My guess for HL2 is that the Administrator is on your side, perhaps leading the group of scientists in City-17 that escaped from Black Mesa and organizing the resistance to the Combine.