Preheat oven to 400ºF (200ºC). Turn each pumpkin on its side, and use a sharp knife to slice the top, including the stem, off the pumpkin, about 1-inch down. Use a spoon to remove the seeds and discard or reserve for another use.
Rub the inside of the pumpkins with 2 tablespoons of the olive oil. Season with salt and pepper then arrange the pumpkins and cut off tops on a baking sheet and roast until the meat is soft, about 35 minutes. Set aside.
Heat a large sauté pan over medium-low heat, add the remaining olive oil and heat through. Add the ground turkey to the pan and sauté, stirring occasionally and using a wooden spoon to break up any large pieces, until the meat is browned, about 5 minutes. Season with salt and pepper then remove the turkey to a large mixing bowl and set aside.
Return the pan to medium heat, add 2 tablespoons of the butter and allow to melt. Add the carrots, onion and celery and sauté until tender, about 5 minutes. Remove to the bowl with the turkey. Add the stale bread and toss to combine.
Lower the oven to 350ºF (180ºC).
Return the pan to medium-low heat, add the remaining butter and allow to melt. Add the sage to the pan and cook until the butter is nutty and fragrant, about 4 minutes.
Pour the butter over the bread and meat mixture. Pour the stock over top, tossing to combine until all the bread is moistened then add 1 cup of the Gruyère cheese and toss to combine.
Fill the roasted pumpkins with the stuffing mixture until packed and almost overflowing. Top each with the remaining Gruyère and bake until the cheese has melted and begins to bubble, about 15 minutes.
Remove and serve immediately with the pumpkin tops propped against the side.