My theory on G-man, his employer and Combine: Assuming that the G-Man has an employer, the current employer was either enslaved by Combine previously and seeking revenge whenever possible and with whatever means (using the Resistance and does not really care about humanity or whatsoever), OR they are almost as powerful as Combine, which wanted to impose thier dominance over Combine. One thing for sure is that the G-Man's employer is much cleverer and cunning than the Combine. Combine fights the wars wherever they go, invest and invent new combined technology for another wars (doing all the dirty works by themselves), while the G-Man's employer only use whatever resources that were there (the Resistance) and hire key service (Gordon) to counteract with Combine. G-Man's employer is either more cost effective, cost concious or they simply do not have that many resources as the Combine does. G-man is a "freelancer", is he? He might have an "employer", but is he REALLY a "freelancer"? Judging to the technology G-man possess (phasing in and out of space and manipulating time), it is hard to believe he is alone or working alone. He could be part of the more powerful races in the universe than the Combine and his current "employer". By leasing out service (Gordon), it meets the "employer" target and most importantly achieving G-Man's own objectives. There may be at least three powerful races in the universe struggling to impose their dominance: G-Man's race, his "employer" and Combine. For G-Man's race to fight two powerful races is not cost effective and may be defeated along the way. So, G-Man's race, while posing as a neutral or "business" party for both races, they actively create conflicts behind the scenes to deplete both races' resources. Thus, G-Man's "employer" is fighting with Combine from planet to planet. Meanwhile, G-Man's race closely watches all wars to select the talented resources (Gordon) to serve as their vehicle (service leasing). When both of the powerful races depleted their resources, G-Man's race will come to front and dominate the entire universe. Earth is just a callateral casualties to their games/wars. All in all, G-Man's race is the cleverest race among the rest as they achieve the target without dirtying their hand (at the time being) and with minimum cost (by using "local" resources).