The following deals with some of the theories about the G-Man and his allegiances.
Theory 1[ | ]
Remember that this is a theory.
The G-Man wanted the Black Mesa incident to occur, as he was seen arguing with a scientist, possible telling him to boost the anti-mass spectrometer 105%. Since he was seen governing where the main characters when in each of the three Half-Life games, it can be theorized that Black Mesa was a sort of test for each of the three characters (Gordon Freeman, Shephard, and Calhoun), since each would be directly or indirectly carrying out the G-Man's motives later (not Shephard yet).
Theory 2[ | ]
Again, another theory. The G-man did NOT want the Black Mesa Incident to occur. This is supported by a specific line in the Half-Life 2 script that said that the G-Man was telling the scientist back in Half-Life NOT to boost the spectrometer. Although all acknowledge that the G-Man has been more or less governing the main characters since Half-Life, these theorizers do not believe that the Black Mesa incident served the G-Man's purposes. A possible reason for the G-Man's intervention was to make sure that the portal to Xen was closed. The G-Man most likely hired Gordon because he viewed him as a valuable tool for later on.
Theory 3[ | ]
The final theory is that the G-man is an alien life-form disguised as a human, who is part of the rebellion against the Combine that came to earth to recruit fighters against the Combine. When the G-man came to earth, he tried to stop the resonance cascade but failed. He then took notice of two people involved in the incident. The first was a theoretical physicist who survived against all odds in the Black Mesa facility & an H.E.C.U. marine (Adrian Shepard), who managed to do the same. When he deemed Gordon to be more useful than Shepard, he confined them both in a "slow teleport" (a.k.a. temporal stasis). This event also explains G-man allowing the Citadel explode in HL2E1, since he would get the Combine to earth en mass to defeat them here.
Previous Theories[ | ]
One of the original theories about the G-man was that he was the mysterious administrator of Black Mesa, judging by the Black Mesa logo on his briefcase. This was removed when Half-Life 2 was released. It is now known that Breen, the Combine Earth Administrator was the Administrator of Black Mesa during the Resonance Cascade.
The Bottom Line[ | ]
Most likely, Valve did not have everything planned out at the time of Half-Life. They may have intended the G-Man to be the administrator, but then later thought it would be an interesting twist if he weren’t. However, Valve plans to release more information about the G-Man and the Combine throughout their new episodic content.