While turkey cooks, place reserved turkey neck, heart and gizzard into large saucepan. Add 6 cups water, 3 1/2 cups chicken broth, carrots, onion, celery and bay leaf. Simmer over medium heat until turkey stock is reduced to 3 cups, about 2 hours.
Strain turkey stock into bowl; reserve turkey neck and giblets. Pull meat off neck. Chop neck meat and giblets.
Melt 5 tablespoons butter in heavy large saucepan over medium heat. Add 5 tablespoons all purpose flour and whisk 2 minutes.
Gradually whisk in turkey stock, cream and up to 1 cup reserved turkey pan juices (juices are salty, so add according to taste). Simmer gravy until thickened to desired consistency, whisking occasionally, about 5 minutes.
Add chopped turkey neck meat and giblets; season to taste with pepper.