a gray man?

It's jargon, so it's hard to say.

This looks like a description of an agent who could easily impersonate any character required. (because of his physical attributes, or lack of them: no strong physical characteristics.)

From this brief definition from an unlikely source:

http://www.urbandictionary.com/define.php?term=grey%20man

One can deduce that a "gray man" is a master of disguises whom can infiltrate any organization via the front door with whatever related uniformed disguise while convincingly appearing and sounding like he belongs there.