It comes from an idea that one of our best site members, Tom Eve (Guitarman314) had a few years ago. As it was succinctly put by Johnd248 on Feb 7, 2010:
"There is a very eloquent contributor here named Guitarman. He has a theory which often proves correct: one of the first due companies is not available for a call. Either on another run, at training, out of service, etc. We call it the Gman theory."
Then, also on Feb 7, 2010, another senior member, efd274, said:
"G'Man is an abbreviation for the Guitarman who is a well versed contributor to the site. Over the course of the last several months there have been many occassions where first alarm units particularly first due Engines or Trucks have not been available for working fires (10-75's and above). The G'Man noted that if you wanted to buff you should go to the house where a unit is at the Rock or on medicals etc since that is where the action will be. It is "officially" now called the G'Man theory by several of the regular contributors to the site including myself. It hasn't been added to the Department's Operational Reference though!!
Hope this answers your question . . . and welcome to the site! And the Gman theory continues to prove itself every day.