How did the Gman get his name?

The title really says it all, how did the Gman in the Half Life series get his name? I'm sure none of the characters in the Half Life series' names the Gman as the Gman so has the name been made up by someone so that everyone knows who they are talking about when really the Gman doesn't have a name? 

I hope that made sense :L Thanks

garry newman (original creator of gmod)

we need a +1 button on this website