In a heavy-bottomed pot, melt the butter over medium heat. Add the flour and mix well to make a roux. Cook, stirring frequently, until the mixture is pale yellow, about 5 minutes.
Whisk the stock into the roux a little at a time, allowing the roux to absorb the liquid before adding more (this will help avoid lumps).
Add the salt, pepper, carrots, and celery, along with rosemary and sage. Let simmer while tasting for herbs and remove rosemary and sage when the gravy has absorbed sufficient flavor.
Simmer till carrots have softened and soup has thickened to coat the back of a spoon. Approx 30-40 minutes.
Add cooked turkey and gnocchi and gently simmer for an additional 5-10 minutes.