my theory on G-man is that he is a recruiter for aperture science and that the reason he was at black mesa is because he was looking for the best scientists
aperture science and black mesa were both competing for government contracts on portal tech and i think aperture science won and the G-man was sent to salvage some scientist for future use by "some government" and destroy black mesa (thats why he gave Eli the crystal)
also the G-man talks funny because he fell into a fire face first and inhaled and burned his larynx
DISCLAIMER: my theory (like most) has no real proof so it most likely wrong and completly stupid
aperture science and black mesa were both competing for government contracts on portal tech and i think aperture science won and the G-man was sent to salvage some scientist for future use by "some government" and destroy black mesa (thats why he gave Eli the crystal)
also the G-man talks funny because he fell into a fire face first and inhaled and burned his larynx
DISCLAIMER: my theory (like most) has no real proof so it most likely wrong and completly stupid