One of the first things that the survivors will have to address with Gregory is the presence of Maggie and Daryl. Negan doesn’t know that Maggie is alive. Daryl’s escape from Sanctuary is sure to make Negan a little upset.
Gregory could kill their plans to attack Negan before they even get started. If he turns Daryl over to Negan it would almost certainly mean that Negan would kill Daryl. And if Negan finds out that Maggie has been at Hilltop all this top and is still alive he may kill her too. Or, he might kill Carl or someone else close to Rick to punish him.
It’s critical that Negan not find out that Maggie is alive until the survivors are ready to attack. Negan will probably send Dwight and other Saviors out to find Daryl. If they find him at Hilltop that will put Hilltop at risk. So Gregory is probably not going to cooperate with Rick willingly because he won’t want to take that risk.
This photo looks like the group are going to be involved in some serious discussions with Gregory about Hilltop’s role in the war. Rick’s great at making speeches, but that doesn’t mean Gregory will go along with Rick’s plan.