Dave Evans and Liam O'Connor are [likely] agents of American Intelligence.
Other than the titles of three adventures listed below, I as yet know nothing about these two gentlemen besides their names.
I can surmise since their activities are recounted in the publication that they were, both these men are agents in very good standing (as in the Ace) with some department of the U.S. government (the G-Man part). The G-Man could point to their working for the FBI since that is the nickname often associated with the people from the Bureau but in truth it could be and has been used to denote "government man" so it could be any department really (except the Treasury which prefer to go by T-Man).
They are entered into the compendium solely based on the titles given to the first two adventures. The Spy Diver definitely sounds spy-related, though the 'diver' part is a curiousity. Dictators For Breakfast sounds like something a harden, or cocksure, operative might brag he regularly consumes. The last tale, The Dead Don't Need Defense, is sufficiently vague, albeit true, as to give or take nothing. Until I learn more, I stay with the two fellows being in the spy business.