Preheat oven to 350 degrees F.
For the pork: Rinse pork loin to remove residue and pat dry with paper towels. Season with salt, pepper, and garlic powder. Heat grapeseed oil over medium-high heat in an oven safe pan, preferably with a lid. Sear pork loin on all sides, leaving each side undisturbed for the first few minutes before turning, to allow the seasonings to integrate into the surface of the meat. Cover the pan and transfer to the oven. Roast, covered, until fork tender, about 1 1/2 hours, or until the pork reaches an internal temperature of 160 degrees F on an instant-read thermometer. Remove cover and increase oven temperature to 425 degrees F for about 10 minutes to allow outside of roast to brown. Remove from oven and allow to rest and carryover cook to 170 degrees F. Slice.
For the marmalade: While the pork is in the oven, make the marmalade. Heat grapeseed oil over medium heat in a small skillet. Add onion and reduce heat to low. Allow to slowly caramelize, about 30 to 40 minutes. Stir in ketchup and salt and pepper, to taste, and allow to cook for another 10 minutes to reduce and thicken. Add additional oil before serving if the marmalade looks too pasty.
Serve marmalade over sliced pork.