For the turkey: Remove any packets or remaining parts inside the turkey. Pat the turkey dry inside and out with paper towels.
Combine the salt and baking powder in a small bowl. Sprinkle the salt mixture inside the turkey cavity lightly, then sprinkle liberally all over the outside. (The turkey should be well coated but not fully encrusted.) Discard any excess. Refrigerate, uncovered, for 24 hours.
When ready to roast, let the turkey come to room temperature for 1 hour.
Meanwhile, heat the oil in a small saucepan over medium-low heat with the poultry seasoning and garlic. Cook on low for 5 minutes, then turn off the heat and let cool completely. Discard the garlic.
Preheat the oven to 425 degrees F.
Set the turkey on a rack set in a stovetop-safe roasting pan. Add the thyme, rosemary, sage, lemons and orange to the cavity. Tie the legs with butcher's twine and fold back the wings for even cooking. Reserve 2 tablespoons of the infused herb oil, then brush the turkey all over with the remaining oil.
Roast the turkey 30 minutes, then reduce the oven temperature to 350 degrees F. Continue to roast, basting occasionally with the reserved oil after 1 hour and spinning the roasting pan halfway through, until the thickest part of the thigh reaches 165 degrees F, 1 hour 30 minutes to 2 hours more. In the last 30 minutes, baste with the pan drippings. Let the turkey rest for 30 minutes before carving.
For the gravy: Set the roasting pan over medium-high heat and bring the drippings to a boil. Sprinkle over the flour, whisking constantly. Continue to whisk the flour while it cooks for 2 to 3 minutes, scraping up any brown bits on the bottom of the pan. Slowly add the chicken stock, stirring constantly, until the gravy is thick and light brown. Remove from the heat and serve immediately (see Cook's Note).
I usually do not need to add salt to the gravy since the turkey is dry-brined.