There's a mole in Rick's army and the evidence for the top candidate dates back to the middle of The Walking Dead's seventh season.
Fans began speculating Father Gabriel would be a mole on The Walking Dead when he was trapped in a trailer with Negan in the Season 8 premiere and never made any effort to turn and shoot the villain. Many suspected the holy man was working with Negan and the reason he is going to "s---" his pants is a result of Negan's plan to free them from their current scenario of being surrounded by walkers.
Now, the evidence of Gabriel being a mole has been furthered.
When Gregory returned to the Hilltop, he pleaded with Maggie for entry, but in doing so promised Negan was fully aware of Rick's plans before he arrived. Somebody, other than Gregory, told Negan that Rick was going to attack the Sanctuary.
The Saviors' preparations were evidenced when King Ezekiel's group was ambushed by a group which was expecting them, using guns which were moved from where Rick expected to find them. Negan's group has some clear intel regarding Rick's plans, some of which Gregory likely wouldn't be aware of, which lends validity to his claim, which is needed seeing as the character is often a cowardly liar.
In Season 7, Father Gabriel appeared to have been taken hostage by Jadis and the Scavengers. It looked as though he was taken from Alexandria with an unidentified person in his car. When Rick arrived at the heap, he was greeted by a very-changed Gabriel who was singing the Sheriff's praises and insisting the new bunch of characters support their cause against the Saviors. Ultimately, Rick would fetch guns for this group only for those guns to be used against them.
Now, Rick is likely heading back to Jadis' group for some conversation cryptically teased between him and Daryl at the end of Sunday's Monsters episode.
With showrunner Scott Gimple promising an episode in the first half of Season 8 which will explain "how some things came to be," a flashback sequence revealing Father Gabriel's true deeds seems more and more likely. In it, fans would see the events which transpired between Gabriel and Jadis prior to Rick's group finding them. Furthermore, Negan and Jadis had already been acquainted and Jeffrey Dean Morgan's villain was in attendance of the original meeting from the beginning, seeing as the Scavengers had been spying on Alexandria well prior to their introduction. It's a complex twist but one which would catch most fans off-guard.
Speaking to ComicBook.com, Father Gabriel actor Seth Gilliam debunked the theories of Gabriel's betrayal. "I just think Gabriel's a bad shot," Gilliam said. "I think that's about as deep as it gets. He couldn't hit the ocean if he was standing on the side of a boat and leapt. That's alright. It just means that he can't shoot, it doesn't mean that he's in cahoots. I've heard some things about in cahoots, which would be an interesting story twist, by the way in an alternative universe, but I don't really see that happening."
Is Gilliam throwing the fans for a loop with Father Gabriel in cahoots with the bad guys or is their still hope for Alexandria's priest?