I am not sure if this is considered true giblet gravy, but here is how I make my gravy.
I throw all the giblets (all the things you pull out of the turkey including the neck) into a pot of water. I toss onion and celery and carrots in. You don't need to chop them, I just quarter the onion. Toss some parsley, salt and pepper in and let it simmer. I generally let this simmer for a couple of hours, sometimes adding a cup or so more water during the cooking time.
I then use the liquid in the roasting pan with the drippings from the turkey.