Christian Gunn is an agent with the CIA.
That is close to all we know about this operative. The two adventures we have of him are short and therefore the amount of intelligence on him is understandably limited.
We know that he, by his own statement, has worked largely in the Middle East and Europe. He indicates that he had worked with a particular French agent "many times in the past" which clearly shows that he has been active for quite a while which would put his age in the thirties, likely the upper half.
Gunn is not described much but we are told he has a thick mat of hair on his chest, has "powerful muscles" in his shoulders and his legs are also very muscular so it is apparent whatever his age is, he takes good care of himself, minus his smoking habit. He is also quite confident in himself as in his talking about a fellow agent who was killed in the line of duty and stating that the agent "made a mistake. I won't make the same one".
He prefers the .9mm Luger worn in a shoulder holster. He sleeps in the nude and is not the least bit hesitant to open his hotel room door to a knock undressed like that wearing only his watch. Holding his Luger at the ready, of course.
Gunn is most definitely a ladies man. In each of the two missions we follow, he meets a sexy woman and becomes intimate with her. He does not express any dissatisfaction with them nor they with him. It is obvious, though, that it is purely physical because there is no sadness expressed when both of them, for different reasons, depart this life shortly after getting to know him.