My theory for the Headcrab's "reproduction" cycle is that The Headcrab is released from the Gonarch's "egg sack" then grows into an infant. The infant then consumes a host, staying on it until it is a youth. It then "melds" itself within the host as the host completes it's transformation into...
My G-Man Theory (Never heard anyone else say it before).
First of all, you NEVER hear anyone say A Gman, or THAT ONE Gman, always THE Gman THE Gman. Since G in Gman stands for Government, his name is The Government Man. THE. He is THE Government man. So, that leads me to think that, no he's not...