Whisk together the water, orange juice, 145g salt, light brown soft sugar, garlic granules, oregano and thyme in a large glass or ceramic bowl. Submerge the pork chops in the brine. Cover the bowl with cling film and brine in the refrigerator for 1 hour.
Heat grapefruit juice, 4 tablespoons dark brown soft sugar and red chilli flakes in a saucepan. Cook and stir until sugar is dissolved and mixture comes to the boil; reduce heat and cook until syrupy, about 15 minutes. Remove 3 tablespoons of the mixture and set aside. Add half of the grapefruit sections to the syrup mixture and cook for 5 to 10 minutes.
Preheat the grill and set the rack about 15cm from the heat source.
Remove the pork chops from the brine and pat dry. Place the pork chops on a baking tray lined with aluminium foil. Brush each side of the chops with the reserved grapefruit juice syrup. Season with salt and pepper. Grill for 7 to 10 minutes on each side and the pork is no longer pink in the centre. An instant-read thermometer inserted into the centre should read 70 degrees C.
Place the remaining fresh grapefruit sections on the pork and drizzle relish over the top.