Giblet Stock: In a stock pot, combine the giblets, neck, water, onion, celery, carrot, bay leaf, and thyme. Over high heat, bring to a simmer, then reduce the heat to medium-low. Simmer, uncovered, until the stock is flavorful, about 2 hours. Strain stock through a fine-mesh strainer and into a large bowl, reserving the giblets and discarding the neck. Finely chop giblets and set aside. Set stock aside to cool.
Place the roasting pan with the turkey drippings across two burners over medium heat. Stir with a wooden spatula, scraping up any browned bits, or "fond," from the bottom of the pan; if necessary, add water to help deglaze the pan. Strain the liquid from the pan through a fine-mesh strainer set over a bowl. Set aside to allow the fat to rise to the top.
Gravy: First, make a roux. In a saucepan over medium-high heat, add 2 tablespoons of the fat from the strained drippings. Sprinkle in the flour and whisk until the flour is incorporated and no longer raw, 1 minute. Now add the stock, about a ½ cup at a time, whisking continuously and making sure it's completely absorbed before adding more. Small additions of liquid and continuous whisking are the keys to smooth gravy. Whisk in the pan drippings, then taste and adjust seasoning as needed.
Finally, add cream and chopped giblets; stir, taste, and adjust seasoning again. Keep warm until ready to transfer to gravy boat or bowl.