To make the stuffing, place gluten-free bread in a food processor and blitz until the mixture resembles crumbs. Transfer to a large bowl and set aside.
Melt butter in a frying pan over medium-high heat, add onion and macadamias and cook, stirring occasionally, for 3-4 minutes or until onion has softened. Allow to cool slightly then transfer to the bowl containing the breadcrumbs.
Add remaining stuffing ingredients to the bowl and stir to combine.
Pat pork loin dry with paper towel, then place on a wire rack in the kitchen sink. Pour a jug of boiling water over the rind, then use paper towel to pat dry.
Preheat oven to 220°C. Lay pork, skin-side down, on a chopping board. Spoon stuffing mixture down the centre of the pork, then roll firmly and tie securely with kitchen twine.
Rub skin with oil and sprinkle generously with salt. Place pork onto a rack in a roasting pan. Roast for 30 minutes. Reduce heat to 180°C and cook for a further 1 hour and 30 minutes (allow 45 minutes per kilo) or until cooked through. Transfer pork to a large plate for serving.