Preheat oven to 400°F / 177°C.
Make stock: Pour oil in the bottom of a Dutch oven or roasting pan. Add turkey giblets and neck and then onion, celery, and carrots. Sprinkle salt over top. Transfer to heated oven and roast, uncovered, until everything is starting to brown lightly, about 60 minutes.
After roasting, transfer the pot to the stovetop (be careful - the handles will be very hot!).
Add the water and bay leaves and bring to a boil over high heat.
Once the stock is boiling, reduce heat to a low simmer. Cover the pot with a lid and simmer, covered, for 60 minutes. (Adjust the temperature as needed to keep the stock bubbling gently.)
Let the stock cool slightly. Strain out solids by pouring the stock through a fine mesh strainer. (Or you can use a strainer with larger holes, but line it with cheesecloth first).
Make gravy: Melt butter over medium heat in a saute pan or skillet.
When the butter is melted and starting to turn golden, whisk in the flour until no dry spots remain. Cook butter and flour together, whisking constantly, until golden brown, about 3 minutes.
Slowly add 3 cups of stock while whisking. (See note about extra stock.)
Bring to a simmer and simmer until the gravy is thickened to your preferred consistency, 6 to 10 minutes.
Taste the gravy and season with some more salt if needed. See notes below to store and reheat.