To make giblet stock: Place giblets (except liver), neck and water in a large saucepan. (Reserve the liver for another use or discard.) Bring to a boil over high heat. Reduce heat and simmer, skimming and discarding any foam, for 1 hour. Strain through a fine-mesh sieve.

To make gravy: After removing the turkey from the roasting pan, pour any pan juices and fat into a large glass measuring cup and place in the freezer so the fat rises to the top, about 10 minutes. Skim the fat off with a spoon and discard. (Alternatively, pour the pan juices and fat into a fat separator then pour the defatted juices into a large measuring cup.) Add any accumulated juices from the resting turkey to the defatted pan juices; add the reserved giblet stock plus enough chicken broth so the combined liquids measure 5 cups total.

Whisk 1/2 cup chicken broth and flour in a small bowl until smooth.

Set the roasting pan over two burners on medium-high heat. Add deglazing liquid; bring to a boil and cook, scraping up the browned bits from the pan, until the liquid is reduced, about 3 minutes. Add the 5 cups of liquid from Step 2. Increase the heat to high and return to a boil, whisking often and scraping up any remaining browned bits. Boil until reduced to 2 3/4 cups, 8 to 12 minutes. Whisk the reserved broth-and-flour mixture into the roasting pan. Boil, whisking constantly, for 2 to 3 minutes. Remove from the heat and pour the gravy through a fine sieve into a large measuring cup. Stir in herbs, if using. Taste and season with salt (if needed) and pepper.

MAKE AHEAD TIP: Prepare through Step 1 and refrigerate for up to 4 hours.