Preheat the oven to 350°F. Line a standard 12-cup muffin tin with foil baking cups or spray with cooking spray, also spray the foil liners if using.
In a large bowl, whisk together the gluten-free flour, gluten-free oats, sugar, baking soda, baking powder, salt, and cinnamon.
In a separate bowl, whisk together the applesauce, eggs, and vanilla extract.
Add the wet ingredients to the dry ingredients and stir until just combined.
Fold in the diced apple.
Divide the batter evenly among the prepared muffin cups using a 1/4 measuring cup.
Bake for 18-20 minutes, or until a cake tester inserted into the center of a muffin comes out clean.
Allow the muffins to cool in the pan for 5 minutes before transferring them to a wire rack to cool.
Once cooled drizzle melted caramel over the muffins and sprinkle with kosher salt or popcorn salt.