He's a transdimentional contracting agent and broker,serving godlike individuals with his own godlike powers. His mortal enemies are the vortigaunts,and does his best to avoid them. He's also the most active and direct of his organization,directly contacting three individuals (Eli Vance,Gordon Freeman,and Corporal Shepard) within a 20 year timeframe,and ensuring his investments pay off (re-arming the nuke in Opposing Force,putting Gordon in stasis for future application,giving Eli the sample that catalyzed the entire series),and doesn't care if he's spotted (Let's play a game...How many times can you spot the G-Man throughout the entire series and all of it's expansions?) because he knows it won't affect things.
That enough of a fank wank? Or do I need to go deeper?