Wash the 4 medium Gala apples and 4 medium Granny Smith apples thoroughly under cool running water. Remove the cores and cut each apple into quarters, leaving the skins intact. Peel the 1 medium navel orange, remove any seeds, and cut it into quarters. Aim for similar-sized apple pieces so they soften evenly during slow cooking.

Place the prepared Gala apples, Granny Smith apples, and navel orange into the 6-quart (5.7 L) slow cooker. Distribute the fruit evenly across the bottom. Add the 3 whole cinnamon sticks (about 3 inches / 7.5 cm each) and 1 teaspoon (about 2 g) whole cloves, allowing the spices to settle between the fruit pieces.

Sprinkle 1/3 cup (about 70 g) packed light brown sugar over the fruit. Pour in 7 cups (1.66 L) water. Stir gently with a wooden spoon to distribute the sugar, cinnamon sticks, and whole cloves. The fruit should be mostly submerged, with enough room beneath the rim for safe simmering.

Cover the slow cooker with its lid and set it to LOW. Cook for 6 hours without regularly lifting the lid, which allows the heat to remain steady. By the end of this period, the apples should be extremely soft, the liquid should have a golden-amber color, and the cinnamon and clove aromas should be noticeable.

After 6 hours on LOW, carefully lift the lid and press a piece of apple against the side of the slow cooker with a wooden spoon. It should collapse with very little resistance. If the apple pieces remain firm, continue cooking on LOW until they soften before proceeding.

Use a potato masher to gently crush the softened Gala apples, Granny Smith apples, and navel orange directly in the slow cooker. Press the fruit several times until most large pieces have broken into a coarse pulp. Keep the masher beneath the liquid as much as possible to prevent splashing. Mashing releases additional apple juice and increases contact between the fruit pulp and the hot liquid.

Replace the lid and continue cooking on LOW for 1 additional hour. The liquid should become slightly darker and more aromatic as the mashed fruit infuses it. Avoid vigorous stirring because excessive agitation can push more fine pulp into the liquid.

Position a fine-mesh strainer over a large heatproof bowl or pitcher. Carefully ladle the hot mixture into the strainer in batches, allowing the cider to drain naturally. Press the softened fruit gently with the back of a spoon to extract more liquid, then discard the apple and orange pulp, 3 cinnamon sticks, and whole cloves. Strain a second time if a smoother texture is preferred. Serve the cider warm, or cool it promptly and refrigerate for serving chilled.