Negan's plan to attack people with walker blood was inspired by Eugene, but it might have also been partially influenced by the ailing Father Gabriel. The priest's illness is taking a major toll on him on The Walking Dead and it almost seems like Father Gabriel is becoming a walker. Although The Walking Dead has never shown a person turn while they are still alive, the showrunners have made it clear that the science behind the virus is very much unknown. Gabriel took some antibiotics, which seemingly halted his infection. But as Season 8 goes on, viewers will see if his sickness takes away more than his eyesight.
Throughout the eight seasons of The Walking Dead, people have only turned into zombies after they have died. If you get a walker bite, you'll die sooner — and thus, turn sooner — but the prerequisite has always been death. (That's why it's also referred to as reanimation.) Yet, something is going on with Father Gabriel that viewers haven't seen before. His illness is extreme.
What fans can reason is that Gabriel probably became infected after covering himself in walker guts back in "The Big Scary U." Without treatment, the infection was getting worse and worse until he found the antibiotics in "Dead or Alive Or" — and if he hadn't taken the medication when he did, he absolutely would have died and turned. This zombie-borne illness also seriously impaired his eyesight and so even though he seems to be on the mend(ish), he'll most likely never fully regain his vision. But there's still something severely off about Gabriel and his eyes do kind of look walker-esque. So is it possible that he's turning into a walker while he's still alive?
Since there's no precedent for someone turning into a zombie while living in the TV show or comics, there's no way to be sure. And it would seem like a major leap — even for a series about the zombie apocalypse. (Also, it should be noted that this is not Gabriel's fate in the comics.) Yet, Negan has kind of hinted that it's a possibility. In "The Big Scary U," Negan noted that he has seen people get sick from cloaking themselves in walker guts. Negan didn't specify what kind of sickness those people had gotten — or whether it prematurely turned them into zombies — but his comment foreshadowed Gabriel's illness.
Then, when Negan announced his plan to attack the Hilltop with walker blood in "Dead or Alive Or," he insinuated that people could turn from this method of biological warfare. After he covered Lucille in walker guts, he said, "With this, it can just be a touch. Or a big, wet kiss. Either way, this gets you full membership and that's what we want — we want people to join the club. Hilltop's gonna learn to tow the line one way or another. Dead or alive — or some kind of sh*t in between."
While you could assume Negan meant that people would get sick and die from the blood and guts and then turn into walkers, perhaps he meant that death doesn't necessarily have to be a part of the equation. After all, he has seen something when it comes to zombie guts that Rick's group hasn't experienced before. If he has seen people become walkers after gutting up, then that could be what's happening to Gabriel.
As for Gabriel's fate, he's nowhere to be found in the preview of the March 25 episode, "Do Not Send Us Astray." And he's stuck with Eugene at the Sanctuary, so if he is going to turn, he'll probably take out Saviors first, which isn't really a bad thing. So while it seems more likely that Gabriel is just sick from the disgusting walker matter that he put on his body, you can't completely rule out the idea that he's becoming a walker. If that is what's happening though, then you better hope that Negan's biological warfare plan keeps getting delayed because the last thing the Hilltop needs is their people turning into walkers.