In a medium bowl, whisk together gluten-free flour, eggs, milk, and a pinch of salt until you achieve a smooth consistency. Let the batter rest for about 30 minutes.

Heat a non-stick skillet over medium heat, lightly greased with butter. Pour a small amount of batter, swirling to coat the bottom. Cook for 1-2 minutes until golden brown, then flip and cook for another 1-2 minutes. Keep warm.

In a large skillet, melt 1 tablespoon of butter over medium heat. Sauté leeks until soft, then add mushrooms and cook until golden. Stir in shredded chicken and heat through.

In a medium saucepan, melt 3 tablespoons of butter. Whisk in flour and cook to form a roux. Gradually pour in warm milk while whisking until thickened, about 5-7 minutes.

Preheat oven to 350°F (175°C). Pour a thin layer of béchamel in a baking dish. Fill crepes and roll them, then arrange seam side down in the dish. Pour remaining béchamel over and sprinkle with Gruyere. Bake for 20-25 minutes.