I think "government man" covers it quite nicely. The combine has its interdimensional "thing" while the GMan is working his own ends in conjunction with "other entities" He knew what was going on at black mesa and just how to bend those events to advance his own agenda, and came out of it with the capability to warp space and time and travel through portals; behind glass through most of HL1... until the end, then hes just kind of there. I swear ive caught glimpses of him in HL2, like in the water craft level I saw him up on the dam...
As smart as gordon is, a scientist and all, he is used by the gman as quite the blunt instrument in the HL2 series.
As smart as gordon is, a scientist and all, he is used by the gman as quite the blunt instrument in the HL2 series.