In a small bowl, combine the gluten-free flour, granulated sugar, baking powder, baking soda, fine sea salt, and ground cinnamon.
In a medium mixing bowl, whisk the buttermilk and eggs together. Then whisk in the flour mixture until well combined.
Whisk in the melted butter, and finally, fold in the diced apple. Set the batter aside to rest while you make the streusel.
To make the streusel, combine the gluten-free quick cooking oats, light brown sugar, and ground cinnamon in a small bowl. Drizzle in the melted butter and stir until it is combined well.
Heat a griddle or non-stick frying pan to a medium high heat. Grease the pan if needed, then pour the batter onto the hot surface in 4-to-6-inch circles.
Sprinkle 1 tablespoon of streusel on top of each pancake while the batter is wet.
Allow the bottom of the pancake to brown and flip to brown the other side. The streusel will caramelize and become crispy when cooked. Continue until all the batter is used.
You can keep the pancakes warm on a baking tray in a 200°F oven until you are ready to serve.
In a small cup or bowl, combine the powdered sugar with milk or water and stir until it forms icing, and no lumps remain.
To serve, top the pancakes with diced apple, remaining streusel, and drizzle with icing.