After you roast the turkey, set the roasting pan with the turkey drippings on a medium-high heat on the stove-top.
Add the stock to the pan, then stir and scrape any turkey bits from the bottom of the roasting pan.
Strain and pour this gravy liquid into a bowl and place it in the fridge for about 15-30 minutes to let the fat float and harden to the top.
Remove hardened fat with a spoon and place the stock into a saucepan. Add in the garlic, sage, and thyme.
Place the saucepan over medium heat and bring to a simmer. Continue to simmer until the stock is reduced by 1/4, or until about 3 cups of stock remain.
Meanwhile, in a small bowl whisk milk and cornstarch until well combined.
Pour milk mixture into the stock, stirring slowly.
Continue to simmer and stir frequently until stock thickens about 3 to 5 minutes.