Victor 'Vic' Gunn is an agent with British Intelligence.
So is Simon Kane. [Note: Why Gunn gets title credit and not Kane is described at the end of this entry.]
When we meet him, he is described as "ace undercover agent to the government". Exactly which department he worked for or to whom he answered is not stated but in the first adventure he notices, looking out the large windows in his luxurious West End flat, that a beacon light atop one of the skyscrapers had gone out. To Gunn it was "an urgent call to action" as he states that was the signal he was needed. Responding to the summons, Gunn arrives in his fancy sports car (driven by his servant, Barrel, whom he immediately sends away) as a drabby shop in a "dingy side street". Entering the facility, he steps into a telephone booth (remember those?) marked out-of-order, presses a button, and is given access to a section of the building no one knew about. It is in a windowless room in that section that he meets with his chief and is given his orders.
Gunn is shown to be a tall, slender man likely in his late 30's with short light brown hair. By the scant look at his apartment it would appear he is with well off on his own or makes very good money. Certainly he has enough to be able to employ Jim Barrel, "a mighty muscled West Indian from Trinidad", who works as his manservant but is really his partner in his cases. Gunn is clearly the senior partner and the man who comes up with the plans but there is no denying that Barrel is a handy man to have around.
Gunn keeps himself "at the peak of physical fitness" which is a very good thing since he is constantly being forced to engage in hand-to-hand combat with multiple opponents at the same time. For this reason, he and Barrel engage in sparring practice with each other whenever time permits. It is said in one adventure that Gunn "was a walking bag-of-tricks", carrying "all manner of innocently disguised gadgets in case of emergencies and was highly skilled in their use". One of those is a ring on his right index finger from which springs a needle with a small amount of knockout drug in it.
Gunn will need all his skills and trick items when he and Barrel learn of power-hungry Baron Rudolph's plan to use a stolen weapon that can render large groups of people unconscious. Rudolph sees himself being the next Oliver Cromwell and wants the job of Dictator. He will succeed in getting it. That will leave Gunn to start a Resistance movement to oust the man.
Now to Simon Kane.
Simon Kane IS Vic Gunn, except for the name. The adventures of Vic Gunn are the adventures of Simon Kane, just told in a different publication several years later and changing all uses of the name 'Gunn' to be 'Kane'. Other than that, no difference.