Make the meatballs: In a large mixing bowl, whisk together the egg and stock until smooth and well-combined. Add the stuffing mix, stir to combine, and set aside to let the stuffing soften.
In a medium skillet, warm the olive oil over medium heat. Add the onion, celery,½ teaspoon salt, and ¼ teaspoon black pepper and cook, stirring occasionally, until tender, 8 to 10 minutes. Add the garlic, sage, poultry seasoning, and 2 teaspoons of water and cook, stirring constantly, until tender, about 2 minutes. Scrape the mixture into the bowl of softened stuffing mix and chill in the refrigerator until the sautéed vegetables are completely cooled, about 10 minutes.
When cool add the ground turkey, grated parmesan cheese, and remaining ½ teaspoon salt and ¼ teaspoon black pepper. Use wet hands to mix until well-combined.
Since the seasoning of the meatball mix will depend heavily on the brand of stuffing you use, cook off a small tester patty of the mix and taste for seasoning. Adjust the meatball mixture with more salt if needed. Using wet hands, form 30 small meatballs the size of a ping pong ball (about 2 tablespoons each), placing the meatballs onto a large sheet pan covered with foil (sprayed with oil) as you form them.
Arrange a rack in the upper third of the oven and turn on the broiler. Broil the meatballs until the top and sides are browned, about 5 to 6 minutes, careful not to burn. Set aside.
Make the gravy: Meanwhile, in a large pot over medium heat, combine 3 ½ cups of the turkey or chicken stock with the sage, thyme, salt (use less if your stock is salted) and black pepper to taste and bring to a boil. Reduce the heat to low, cover with a lid, and simmer for 15 minutes. Discard the herbs.
Meanwhile, in a blender add the remaining ½ cup of cold or room temperature stock with the flour and poultry seasoning and blend until smooth. Add 1 cup of the hot stock into the blender to temper and blend again, then pour into the pot with the simmering stock, whisking until thickened, 2 minutes. Add the butter and whisk until melted, taste for salt and adjust as needed.
Gently transfer the browned meatballs into the simmering gravy and cook, covered, for 20 minutes, stirring occasionally, uncovering the last 5 minutes, to thicken.
Transfer the meatballs and gravy into a large serving bowl or platter, garnish with parsley, and serve hot.
Notes
If you want to make this faster, you can use store bought gravy or a gravy packet.
I haven't tested but I think this would be fine to use your favorite GF stuffing and flour, like Cup4Cup.