I think G-man is just a government man who got stuck in with the wrong crowd.
But seriously, I have no idea. He never really seemed important 'till the end of Half-Life 2, before then he was just there talking for me.
it’s just my opinion that gordon it’s not his agent , but his client , he is like a lawyer that represents gordon in front of the begings with most importance ever
I think the theory I hate most is that “The G-man is Gordon Freeman cause of Gordon Freeman”
This theory is stupid
Why would gordon freeman allow his past self to almost die!?
Your sig really cheered me up after re-registering 
coz he already finished the game in his time and know he is going to make it
lol
good theory
And yet there are still holes. For instance his facial structure is different.
the difference in only in your eyes
The resemblance is also only in your eyes.
Also reposting old theory:
The reason why you always pop up in places at the perfect timing is because of the G-Man. He stalls or hastens you whenever it is needed. Perfect example would be the bridge in Episode 2. He makes it crumble because Gordon was ahead of schedule.
Proof: was sighted on the bridge, moving towards Gordon’s next objective.
Maybe gordon is some sort of test subject and the earth is the G-Mans “employers” test lab?
Here’s my whole theory on the g-man:
He’s some kind of interdimensional being taking the form he is in b/c it is the Earth’s closest equivolent to his true line of work. He works for some kind of interdimensional mercenary and/or government group. He is the ‘representative’ to this pocket of reality in charge of recruiting possible field agents for his firm. He observes Freeman and becomes his handler, choosing who he works for and when he is needed. Freeman’s contract is available to the highest bidder and somehow the rebels were able to aford it for Half-Life 2. I think that Eli probably had more to do with this than anybody else. When the vorts rescue Freeman and alex from the g-mans clutches at the end of HalfLife 2, part of the contract has been breached and the g-man exacts his revenge with the message implanted through alex. And then Eli dies,the result of the initial contract with the interdimensional firm being broken. I can’t help but wonder what the resistance could have offered that the combine couldn’t.
or alternatively
The G-man is the combined embodyment of Chuck Norris, Russel Crowe, Samuel L. Jackson, and Jet Li. lol.
the biggest reference to gman seems to be from the vortigaunts, although easter egg-ishly cryptic. was freeing them just an inconvenience while directing Gordon to the destroy nihilanth?
I mean, why do the vorts oppose him throughout the episodes if his action resulted in their escape from xen. gman not predicting their presence amongst gordon is about as valid as the portal storm being an unforeseen consequence.
Who do the Vort’s oppose throughout the episodes???
Gman is the Pyro, the estranged husband of GlaDOS and father of Uriah the doctor-gaunt.
Maybe the vorts are just lying to you. They really hate gordon 
*Zapow ninja
I love how the Gordon freeman is GMAN thing can also be
Garry newMAN
or George Costanza + Newman from Seinfield
I love how people seem to think G-man means its an abreviation of his real name.
The funniest thing is G-Man isn’t even an official name. Valve never gave him a name, someone just happened to name the original model gman and it caught on.
g man comes from the termin gman that means gouvernament man ,it’s a therm often used for FBI CIA NSA , whatever , and it’s true , the true gouvernament seem to be gman 's employers , but it’s not his name , it may be George Harrison or something american