Place oxtails, onion, garlic, Greek seasoning, 1 tablespoon salt, 1 teaspoon pepper, and seasoning salt in a large stockpot. Fill with enough water to cover oxtails and place over high heat. Bring to a boil. Cover and reduce heat to medium. Cook oxtails until tender, 2 1/2 to 3 hours. Remove from heat and reserve 2 cups broth.

Warm bacon drippings or shortening in a skillet over medium heat. Add flour; cook and stir for 3 minutes.

Stir in 2 cups reserved broth and browning sauce. Season with salt, pepper, and garlic powder. Cook, stirring constantly, until gravy thickens.

Place oxtails in the skillet; stir to coat with gravy. Cook, stirring occasionally, over medium-low heat until warmed through, about 5 minutes.