why Gregory isn't a threat
Or rather, why he realistically shouldn't be.
In order to go to the Saviors to betray Hilltop, he would have to get there. And likely alone, since I don't see anyone else defecting with him. I don't see Gregory successfully getting to the Saviors. First he'd have to navigate there. Secondly, he'd have to take care of himself and defend himself from walkers en route. I don't really think he's capable of this. He seems likelier to head in the wrong direction and get himself killed in a pathetic manner involving one walker.
Now, I think they'll get him to the Saviors perfectly fine for story purposes, but it'll likely show him leaving Hilltop, and next time we see him, he's arriving at the Sanctuary with seemingly little effort.