“Why won’t you listen, Jack? Why won’t you see what’s happening here?”
This could also be a meme. “God, McCree ordered HOW many hats?”
My personal headcanon is that Gabriel was the one constantly trying to tell Jack that something within Overwatch and Blackwatch was going very, very wrong. However, Gabe and Jack were pitted against each other by Talon’s sleeper agents within the UN, manipulated even. Jack believed (or rather, was manipulated into believing) Gabe to be paranoid, Gabe believed Jack to be ignorant and indifferent towards him.
They didn’t blow up the HQ in Switzerland while fighting. Talon blew it up to kill them because they finally, finally had started talking to each other.