Gabe : "Moira, what the fuck are you doing to me?!"
Moira : "I'm debunking theories that Mercy turned you into a shitty edgy OC."

Given that Reaper still seems to blame Mercy for his current condition in the in-game quotes, it could be a combination of Moira's and Mercy's tech that put him in the rapid decay-regeneration Hell. Something like this, maybe:

> Moira gives Gabe some simple initial shadowy powers, maybe similar to her "Fade" ability.
> Jack and Gabe come to blows, and Gabe is critically injured in the Swiss HQ explosion.
> Angela tries to revive Gabe.
> Thanks to Moira altering Gabe's physiology, the resurrection fails and turns him into the monstrosity known as Reaper.
> Reaper blames Mercy for his current state, even if she was only the catalyst.