I have a personal headcanon that Gabriel grew up in a part of LA that necessitated him looking after his younger siblings and trying to keep them out of trouble—specifically gang-related business.  What if, given that McCree was legally a minor when he was recruited into Overwatch/Blackwatch, the only reason that McCree wasn’t tried as an adult and locked up alongside
the rest of the Deadlock Gang was because Gabriel specifically vouched for him?

Here’s how I see it going down.

Canonically the Deadlock Gang is arrested (instead of, say, being killed in a hail of gunfire) which makes me think that this was more a covert rather than an overt operation.

So Overwatch assigns Gabriel Reyes to infiltrate the Deadlock Gang.  Most of its members are adults, but Gabriel immediately notices this kid too young to grow a proper beard and immediately gravitates to him, being reminded of his own brothers and sisters back home.  Over a few months, as he gathers information about the gang’s movements and their major weapons and drug suppliers, he grows to genuinely like this kid—as he learns on day two, his name is Jesse McCree—and he becomes an unofficial mentor to him, as he was for Morrison before him.  He teaches him soldier’s tricks, how to still his breathing in the middle of a firefight so that the erratic adrenaline shivers smooth out into steady aim, how to fade into the night so thoroughly that you’d swear that he was invisible.

But of course one day he’s ordered to bring the hammer down on the gang, and he does so with typical Reyes ruthlessness and typical Reyes lack of hesitation.  Overwatch agents hit every single supplier of the Deadlock Gang, while Reyes leads the team that takes the Gang themselves.

And the Gang fights back, of course; Reyes is forced to shoot several (though thanks to Mercy wounds that would otherwise kill them just put them down for a while), and most of the Gang aren’t exactly insanely skilled marksmen.

But then Reyes kicks in the door to find McCree with his Peacekeeper leveled at his head—and all of a sudden there’s the gang kid he’d beat up when he was sixteen for threatening his siblings into joining up, his face wide and staring and his lip and one cheek going puffy and one eye blackened and just then he’d seen himself in the kid’s face and looking out of his eyes and Gabriel had seen just then all the little choices that had meant that he’d grown up a decent student in a decent school with a loving family and that had meant that the poor kid he was straddling had joined a gang when he was ten out of a sheer desire to be accepted by someone, by anyoneand just like then, he can’t pull the trigger, he can’t throw that one last punch, he can’t move.  For once Gabriel just can’t bring himself to be utterly ruthless in the execution of his job.

Except that this time, his faith isn’t rewarded by the rest of the gang coming by the next day and riddling the front of his house with bullets.  McCree puts the Peacekeeper down and raises his hands.

And then Gabriel goes to Morrison the next day and pulls every favor he has, arguing with his former student for an hour—he’s a good kid, Jack, he is, he has potential, I can make something of him, I can turn him into a force for good Jack, like I did with you—to make sure that McCree gets the offer that the others didn’t.

Join, he tells McCree, or face trial and lockup.

And again McCree rewards his faith.

And then just imagine the betrayal after the fall of Overwatch, when McCree’s had his time to grieve the loss of his mentor, the man who’d been like an older brother to him, and the loss of the men and women of Overwatch who’d been like family to him, when he sees the hooded, masked figure striding along with a pair of shotguns that he’d last seen in the hands of one Gabriel Reyes.