Just taking a wild guess here, but in the real world of spies and secret agents, if you have done enough to gain a reputation among all the other spies and secret agents, doesn't that mean you aren't very secret anymore?
This book's main character is a guy who travels the world killing people. Apparently he's done that a lot since all the bad guys are aware of him and call him the Gray Man. He finds himself on a mission against a large corporation, and that corporation doesn't appreciate his attempt to hurt them. They array all the world's bad guys against him in an attempt to stop his progress to their location. He survives the gauntlet, reaches his destination, and ends up getting a job offer from the people set on whacking him.
Beyond the preposterous premise, the whole book seems to be a series of mini-episodes one right after the next. He faces a batch of guys from some country who try to kill him, he miraculously defeats them, he gets closer to his destination, and then he runs into another bunch. This is more like a character from a video game, defeating progressively stronger opponents in order to get to a higher level.
There are many action scenes, and most are hard to swallow. Just like in a B action/adventure movie, this guy gets shot, beat up, and cut up, but through shear force of will ends up defeating all those professionals set to take him out. Despite his damage, he moves on to defeat group after group. Eventually this gets old. The hero is supposed to win, of course, but enough is enough.
The only saving grace for this book is the author's prose. He writes well enough that you stay hooked on the story throughout its improbable course, overcoming your urge to press the B.S. button and delete it from your Kindle.