I think the most plausible answer for G is Government since he really really looks like those government employees and gman is slang for government man.
Also to note that is the gman that commissioned the military forces to attack black mesa after the incident (actually he knew the incident would happen).
From Adrian Shephard diary (Half-Life Opposing Force Protagonist)
"March 7 - I finally saw the government guy today. I am not sure he is a g-man, but he was wearing a really uptight suit and carrying a briefcase. He looked more like a lawyer or insurance agent to me. I did notice him checking me out. Several times throughout the day I spotted him just watching me during training. I wonder what he's up to..."