Trim the visible fat from the loin. If necessary, cut the roast to fit into a 3½-, 4-, or 5-quart crockpot. In a small bowl, combine the garlic powder, ginger, thyme, and pepper. Rub the spice mixture over the entire surface of the loin. In a large skillet, heat the oil and brown the loin slowly on all sides. Drain off the fat. Transfer the loin to the crockpot. Combine the chicken broth, lemon juice, and soy sauce; pour over the loin. Cover and cook on a low-heat setting for 8 to 10 hours or on a high-heat setting for 4 to 5 hours. When the roast is done, transfer the meat to a serving platter and cover to keep it warm. To make the gravy, pour the juices from the crockpot into a glass measuring cup. Skim off the fat. Measure 2 cups of liquid, adding water to the juices, if necessary, to make 2 cups. Transfer the juices to a saucepan, reserving ½ cup. Stir the cornstarch into the reserved ½ cup of juice until dissolved, then stir into the juices in the saucepan. Heat, stirring frequently, until the gravy is thickened and bubbly, 5 to 7 minutes. Cook and stir 2 minutes more. Season the gravy to taste with salt and pepper. Slice the roast and serve it with the gravy.