Place turkey giblets and neck into a saucepan with 950ml water and bring to the boil; reduce heat to low. Simmer until stock is reduced to 700ml, about 1 hour. Strain stock and reserve 70g of giblets; chop.
Combine giblet stock and turkey drippings in a saucepan over medium heat. Whisk cornflour in 120ml cold water in a bowl until smooth. Whisk cornflour into stock and drippings. Bring to the boil, reduce heat to low, and stir chopped giblets and hard-cooked egg into the gravy. Season with salt and black pepper. Simmer until gravy is thickened, about 5 minutes.