Combine garlic powder, onion powder, pepper, red pepper flakes, and ginger in a small bowl. Rub mixture all over pork loin.
Heat oil in a pan over medium-high heat. Add pork loin and brown on all sides.
Place pork loin in a 6-quart slow cooker.
In a medium bowl or large glass measuring cup, combine chicken broth, lemon juice, and soy sauce. Pour mixture over pork.
Cover slow cooker and cook on HIGH for 4 to 5 hours or on LOW for 8 to 10 hours.
Remove pork from slow cooker and cover to keep warm. Pour liquid from crock pot into a large glass measuring cup. Skim fat off top and measure out 2 cups of liquid. Pour 1/2 a cup of the 2 cups into a small bowl and the remaining 1 1/2 cups into a small saucepan. If the 1/2 a cup in the bowl is still very hot, let it cool some, and then stir in the cornstarch. Pour cornstarch mixture into saucepan with the rest of the liquid and bring to a simmer, cooking and stirring until thickened. Serve gravy with pork.
Notes
Instead of a pork loin, you could use a couple of pork tenderloins.