Preheat oven to 350°F (175°C). Position a rack in the center of the oven. Line a 9×13‑inch baking dish with parchment paper, leaving an overhang on two opposite sides for easy removal.
In a small bowl, combine 1 ½ cups gluten-free graham cracker crumbs and ½ cup melted butter (salted or unsalted). Stir until the crumbs are evenly moistened.
Press the graham cracker mixture firmly and evenly into the bottom of the prepared baking dish, pressing into the corners to form a compact crust.
Evenly sprinkle the following over the crust: 1 cup semi-sweet chocolate chips, 1 cup butterscotch chips, 1 cup shredded sweetened coconut, 1 cup chopped walnuts or pecans, and ¼ teaspoon salt.
Pour the 14 oz. can sweetened condensed milk evenly over the layered toppings. Gently tilt the pan or use the back of a spoon to help the condensed milk reach the edges and corners so the toppings are well coated.
Bake on the center rack at 350°F for 25–30 minutes, until the edges are set and the top is beginning to brown.
Remove the pan from the oven and place it on a wire rack. Let the bars cool in the pan for 30 minutes.
Using the parchment overhang, lift the bars from the pan and transfer to the wire rack. Allow to cool to room temperature (or until firm) before cutting into bars. Cut into pieces and enjoy.