The gravy thickens because of the roux and the long cooking time. Make sure to have some extra stock to add in case the gravy is thickening more than you want. More wine or even water will work if you don’t have more stock. Using less oil and flour (say ¼ cup each instead of ½ cup) will make a thinner roux, but it’s mostly the cooking time. Make sure to keep the lid on and the heat low while you simmer the chicken. Of course, if you want a thicker gravy, remove the lid or offset it to partially open while the gravy simmers.