Trim the visible fat from the loin. If necessary, cut the roast to fit into a 3 1/2-, 4- or 5-quart slow cooker. In a small bowl, combine the garlic powder, ginger, thyme and 1/4 teaspoon pepper. Rub the spice mixture over the entire surface of the loin.
In a large skillet, heat the oil over medium to medium-high heat and brown the loin slowly on all sides. Drain off the fat. Transfer the loin to the slow cooker. Combine the chicken broth, lemon juice and soy sauce in a small bowl; pour over the loin. Cover and cook on low for 8 to 10 hours or on high 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 slow cooker 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 1/2 cup. Stir the cornstarch into the reserved 1/2 cup of juices until dissolved, then stir into the juices in the saucepan. Heat over medium heat, stirring frequently, until the gravy is thickened and bubbly, 2 to 3 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.