Preheat the oven to 400°. Put the turkey parts in a flameproof roasting pan and roast for about 1 hour, turning occasionally, until well browned. Transfer the turkey parts to a large pot.
Set the roasting pan over 2 burners. Add 1 cup of the water and boil over moderately high heat, scraping up the browned bits from the bottom of the pan. Add the liquid to the pot.
Add the turkey neck and giblets to the pot along with the onion, carrot, celery, garlic, salt, generous pinches of pepper, and the remaining 8 cups of water. Bring to a boil over high heat. Reduce the heat to moderately low and simmer, partially covered, until the meat is falling off the bones and the gizzard is very tender, about 2 hours.
Strain the stock. Finely dice the gizzard and heart and reserve for the gravy. Skim the fat from the stock before using.
Make Ahead: The stock and giblets can be refrigerated separately for up to 2 days; any leftover stock can be frozen for up to 1 month.