My guess is It was Michonne (If they are not totally screwing with us). It's Michonne's POV in the shafts of lights. The view of someone at the back of the trailer with someone in front. Daryl is in front of her, so it's her POV seeing the odd shafts of lights in the trailer side. So it make sense they START with her POV and reinforce that at several points, they END with her POV.
Negan kills someone with no hair hanging in their face (POV shows no hair obstructing their view as Negan strikes). Daryl, Glenn, Rick, Carl and Maggie have hair in their faces. Carl only has one eye as well. So does Eugene since his was swollen from the beating. The others don't, but Aaron, Rosita, Sasha, Abraham, Erin and MICHONNE - they cut her locks away from her face and hung them on the Zombie.
If they are not screwing around it has to be a main character -- the death of Erin, Abraham, Aaron, Rosita, Eugene and Sasha just don't have the impact. Michonne would. Also, Rick has trouble keeping his women alive...lol.
Gimble said the clues were there. He also said it wasn't Glenn, that they were taking a "LEFT TURN" from Glenn's death in the comics. Glenn is on the end of the line -- who is to his left? Daryl and Michonne. Daryl has hair in his face, Michonne doesn't. But they are both to Glenn's LEFT.
Another nail in the coffin that makes me believe it's Michonne -- when Negan strikes he goes "Damn.....taking it like a champ." Negan would likely expect Daryl to take it "like a man". But he might not expect Michonne to do it. He would be impressed by a woman taking it head on. Daryl is weak and bleeding out. One swing from Lucille and he would go down (same reasoning for Maggie and Eugene - they are in rough condition). Abraham would take it head on, but again, you would expect that of him.
Initially, before I saw the episode I took it to mean Maggie and the "Left turn" from Glenn. She was upset about the ending when she read the script. So it fit. But I have a feeling Gimble was TELLING you without telling you look to the immediately left of Glenn in the lineup.
The final thing that points to Michonne. Rick said he was sick when he read the script. (Norman has said he knows too.) Outside of Carl which I ruled out, Michonne is the closest one to him - a character we all love so much. THAT might make him sick.